diff --git a/go/packages/packages.go b/go/packages/packages.go
index 92fce758..f40720d0 100644
--- a/go/packages/packages.go
+++ b/go/packages/packages.go
@@ -15,13 +15,14 @@ import (
 	"go/scanner"
 	"go/token"
 	"go/types"
+	"io/ioutil"
 	"log"
 	"os"
+	"path/filepath"
+	"runtime"
 	"sync"
 
 	"golang.org/x/tools/go/gcexportdata"
-	"io/ioutil"
-	"path/filepath"
 )
 
 // A LoadMode specifies the amount of detail to return when loading.
@@ -681,6 +682,14 @@ func (ld *loader) loadPackage(lpkg *loaderPackage) {
 		panic("unreachable")
 	})
 
+	// This is only an approximation.
+	// TODO(adonovan): derive Sizes from the underlying build system.
+	goarch := runtime.GOARCH
+	if x, ok := os.LookupEnv("GOARCH"); ok {
+		goarch = x
+	}
+	sizes := types.SizesFor("gc", goarch)
+
 	// type-check
 	tc := &types.Config{
 		Importer: importer,
@@ -691,9 +700,7 @@ func (ld *loader) loadPackage(lpkg *loaderPackage) {
 		IgnoreFuncBodies: ld.Mode < LoadAllSyntax && !lpkg.initial,
 
 		Error: appendError,
-
-		// TODO(adonovan): derive Sizes from the underlying
-		// build system.
+		Sizes: sizes,
 	}
 	types.NewChecker(tc, ld.Fset, lpkg.Types, lpkg.TypesInfo).Files(lpkg.Syntax)
 
diff --git a/go/packages/packages_test.go b/go/packages/packages_test.go
index 555a4908..f5e99ba8 100644
--- a/go/packages/packages_test.go
+++ b/go/packages/packages_test.go
@@ -9,6 +9,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"go/ast"
+	constantpkg "go/constant"
 	"go/parser"
 	"go/token"
 	"go/types"
@@ -1041,6 +1042,41 @@ func TestContains(t *testing.T) {
 	}
 }
 
+// This test ensures that the effective GOARCH variable in the
+// application determines the Sizes function used by the type checker.
+// This behavior is a stop-gap until we make the build system's query
+// too report the correct sizes function for the actual configuration.
+func TestSizes(t *testing.T) {
+	tmp, cleanup := makeTree(t, map[string]string{
+		"src/a/a.go": `package a; import "unsafe"; const WordSize = 8*unsafe.Sizeof(int(0))`,
+	})
+	defer cleanup()
+
+	savedGOARCH := os.Getenv("GOARCH")
+	defer os.Setenv("GOARCH", savedGOARCH)
+
+	for arch, wantWordSize := range map[string]int64{"386": 32, "amd64": 64} {
+		os.Setenv("GOARCH", arch)
+		cfg := &packages.Config{
+			Mode: packages.LoadSyntax,
+			Dir:  tmp,
+			Env:  append(os.Environ(), "GOPATH="+tmp, "GO111MODULE=off"),
+		}
+		initial, err := packages.Load(cfg, "a")
+		if err != nil {
+			t.Fatal(err)
+		}
+		if packages.PrintErrors(initial) > 0 {
+			t.Fatal("there were errors")
+		}
+		gotWordSize, _ := constantpkg.Int64Val(constant(initial[0], "WordSize").Val())
+		if gotWordSize != wantWordSize {
+			t.Errorf("for GOARCH=%s, got word size %d, want %d", arch, gotWordSize, wantWordSize)
+		}
+	}
+
+}
+
 // TestContains_FallbackSticks ensures that when there are both contains and non-contains queries
 // the decision whether to fallback to the pre-1.11 go list sticks across both sets of calls to
 // go list.