diff --git a/schematron.sch b/schematron.sch
index 3714397..a744936 100644
--- a/schematron.sch
+++ b/schematron.sch
@@ -21,7 +21,9 @@ THIS SCHEMA HAS BEEN MODIFIED FROM THE SCHEMA DEFINED IN ISO/IEC 19757 3,
 AND SHOULD NOT BE INTERPRETED AS COMPLYING WITH THAT STANDARD."
 -->
 
-<sch:schema xmlns:sch="http://purl.oclc.org/dsdl/schematron" xml:lang="en" queryBinding="xslt2">
+<sch:schema xmlns:sch="http://purl.oclc.org/dsdl/schematron" xml:lang="en" queryBinding="xslt2"
+    xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
+    <xsl:key name='abstract-pattern' match='sch:pattern[@abstract="true"]' use='@id'/>
     <sch:title>Schema for Additional Constraints in Schematron</sch:title>
     <sch:ns prefix="sch" uri="http://purl.oclc.org/dsdl/schematron"/>
     <sch:p>This schema supplies some constraints in addition to those given in the
@@ -33,18 +35,6 @@ AND SHOULD NOT BE INTERPRETED AS COMPLYING WITH THAT STANDARD."
                 attribute of the active element shall match the id
                 attribute of a pattern.</sch:assert>
         </sch:rule>
-        <sch:rule context="sch:pattern[@is-a]">
-            <sch:assert test="//sch:pattern[@abstract='true'][@id=current()/@is-a]" id="abstract-patterns"> The
-                is-a attribute of a pattern element shall match 
-                the id attribute of an abstract pattern.
-            </sch:assert>
-            <sch:assert test="every $name in current()/sch:param/@name
-                satisfies $name eq //sch:pattern[@abstract='true'][@id=current()/@is-a]/sch:param/@name" id="param-names-match"> The
-                name attribute of a pattern param element shall match 
-                the id attribute of an abstract pattern param element.
-            </sch:assert>
-            <sch:assert test="TODO: missing param value"></sch:assert>
-        </sch:rule>
         <sch:rule context="sch:extends">
             <sch:assert test="//sch:rule[@abstract='true'][@id=current()/@rule]" id="abstract-rules"> The rule
                 attribute of an extends element shall match the id
@@ -64,4 +54,20 @@ AND SHOULD NOT BE INTERPRETED AS COMPLYING WITH THAT STANDARD."
                 id="param-name">A parameter name should not use the same name as a parameter in the same scope.</sch:assert>
         </sch:rule>
     </sch:pattern>
+    <sch:pattern>
+        <sch:rule context="sch:pattern[@is-a]">
+            <sch:assert test="key('abstract-pattern', @is-a)" id="abstract-patterns"> The
+                is-a attribute of a pattern element shall match 
+                the id attribute of an abstract pattern.
+            </sch:assert>
+            <sch:assert test="every $name in current()/sch:param/@name
+                satisfies $name eq key('abstract-pattern', @is-a)/sch:param/@name" id="param-names-match"> The
+                name attribute of a pattern param element shall match 
+                the id attribute of an abstract pattern param element.
+            </sch:assert>
+        </sch:rule>
+        <sch:rule context="sch:pattern[@is-a]/sch:param">
+            <sch:assert test="@value or key('abstract-pattern', ../@is-a)/sch:param/@value" id="missing-param-value">A pattern parameter value shall be specified.</sch:assert>
+        </sch:rule>
+    </sch:pattern>
 </sch:schema>
diff --git a/schematron.xspec b/schematron.xspec
index facfa84..d10bfbe 100644
--- a/schematron.xspec
+++ b/schematron.xspec
@@ -39,6 +39,26 @@
             <pattern id='p7' is-a='bar'>
                 <param name="blort"/>
             </pattern>
+            <pattern id='p8' abstract='true'>
+                <!-- abstract pattern param contains default value -->
+                <param name="jam" value='spam'/>
+            </pattern>
+            <pattern id='p9' is-a='p8'>
+                <!-- concrete pattern param missing; use default from abstract -->
+                <param name="jam"/>
+            </pattern>
+            <pattern id='p10' is-a='p8'>
+                <!-- concrete pattern param overrides default value -->
+                <param name="jam" value='clam'/>
+            </pattern>
+            <pattern id="p11" abstract='true'>
+                <!-- abstract pattern param has no default value -->
+                <param name="sham"/>
+            </pattern>
+            <pattern id='p12' is-a='p11'>
+                <!-- concrete pattern param specifies value -->
+                <param name="sham" value='shazam'/>
+            </pattern>
         </schema></x:context>
 
         <x:scenario label="active pattern references">
@@ -88,7 +108,7 @@
                 location="/sch:schema/sch:pattern[@id='bar']/sch:param[1]"/>
         </x:scenario>
         
-        <!-- not-assert version of previous, with no duplicate schema params -->
+        <!-- not-assert version of previous scenario, with no duplicate schema params -->
         <x:scenario label="schema param dupe name">
             <x:context><sch:schema>
                 <sch:param name='pram1'/>
@@ -107,5 +127,20 @@
                 id="param-names-match"
                 location="/sch:schema/sch:pattern[@id='p7']"/>
         </x:scenario>
+        
+        <x:scenario label="pattern param value must be present">
+            <x:expect-assert label="param value not present on concrete or abstract pattern params"
+                id="missing-param-value"
+                location="/sch:schema/sch:pattern[@id='p7']/sch:param"/>
+            <x:expect-not-assert label="param value defaults to abstract pattern param"
+                id="missing-param-value"
+                location="/sch:schema/sch:pattern[@id='p9']/sch:param"/>
+            <x:expect-not-assert label="concrete pattern param value overrides abstract pattern default param value"
+                id="missing-param-value"
+                location="/sch:schema/sch:pattern[@id='p10']/sch:param"/>
+            <x:expect-not-assert label="concrete pattern param specifies value (no abstract pattern param default)"
+                id="missing-param-value"
+                location="/sch:schema/sch:pattern[@id='p12']/sch:param"/>
+        </x:scenario>
     </x:scenario>
 </x:description>