diff --git a/lib/oga/xpath/compiler.rb b/lib/oga/xpath/compiler.rb
index 422ae70..73bdd28 100644
--- a/lib/oga/xpath/compiler.rb
+++ b/lib/oga/xpath/compiler.rb
@@ -1103,6 +1103,28 @@ module Oga
           end
       end
 
+      # @param [Oga::Ruby::Node] input
+      # @param [AST::Node] arg
+      # @return [Oga::Ruby::Node]
+      def on_call_sum(input, arg)
+        unless return_nodeset?(arg)
+          raise TypeError, 'sum() can only operate on a path, axis or predicate'
+        end
+
+        sum_var    = unique_literal(:sum)
+        conversion = literal(Conversion)
+
+        sum_var.assign(literal(0.0))
+          .followed_by do
+            process(arg, input) do |matched_node|
+              sum_var.assign(sum_var + conversion.to_float(matched_node.text))
+            end
+          end
+          .followed_by do
+            block_given? ? sum_var.zero?.not.if_true { yield } : sum_var
+          end
+      end
+
       ##
       # Delegates type tests to specific handlers.
       #
diff --git a/spec/oga/xpath/compiler/calls/sum_spec.rb b/spec/oga/xpath/compiler/calls/sum_spec.rb
index 653facb..19e5cc7 100644
--- a/spec/oga/xpath/compiler/calls/sum_spec.rb
+++ b/spec/oga/xpath/compiler/calls/sum_spec.rb
@@ -4,25 +4,36 @@ describe Oga::XPath::Compiler do
   describe 'sum() spec' do
     before do
       @document = parse('<root><a>1</a><b>2</b></root>')
+
+      @a1 = @document.children[0].children[0]
     end
 
-    it 'returns the sum of the <root> node' do
-      # The test of <root> is "12", which is then converted to a number.
-      evaluate_xpath(@document, 'sum(root)').should == 12.0
+    describe 'at the top-level' do
+      it 'returns the sum of the <root> node' do
+        # The test of <root> is "12", which is then converted to a number.
+        evaluate_xpath(@document, 'sum(root)').should == 12.0
+      end
+
+      it 'returns the sum of the child nodes of the <root> node' do
+        evaluate_xpath(@document, 'sum(root/*)').should == 3.0
+      end
+
+      it 'returns zero by default' do
+        evaluate_xpath(@document, 'sum(foo)').should be_zero
+      end
+
+      it 'raises a TypeError for non node set arguments' do
+        block = -> { evaluate_xpath(@document, 'sum("foo")') }
+
+        block.should raise_error(TypeError)
+      end
     end
 
-    it 'returns the sum of the child nodes of the <root> node' do
-      evaluate_xpath(@document, 'sum(root/*)').should == 3.0
-    end
-
-    it 'returns zero by default' do
-      evaluate_xpath(@document, 'sum(foo)').should be_zero
-    end
-
-    it 'raises a TypeError for non node set arguments' do
-      block = -> { evaluate_xpath(@document, 'sum("foo")') }
-
-      block.should raise_error(TypeError)
+    describe 'in a predicate' do
+      it 'returns a NodeSet containing all matching nodes' do
+        evaluate_xpath(@document, 'root/a[sum(/root/*)]')
+          .should == node_set(@a1)
+      end
     end
   end
 end