diff --git a/compiler/simplCore/Simplify.hs b/compiler/simplCore/Simplify.hs
--- a/compiler/simplCore/Simplify.hs
+++ b/compiler/simplCore/Simplify.hs
@@ -2251,7 +2251,10 @@
 
         ; dflags <- getDynFlags
         ; let alts_ty' = contResultType dup_cont
-        ; case_expr <- mkCase dflags scrut' case_bndr' alts_ty' alts'
+        -- The seqType below is needed to avoid a space leak (#13426)
+        -- but I don't know why.
+        ; case_expr <- seqType alts_ty' `seq`
+                       mkCase dflags scrut' case_bndr' alts_ty' alts'
 
         -- Notice that rebuild gets the in-scope set from env', not alt_env
         -- (which in any case is only build in simplAlts)
diff --git a/testsuite/tests/perf/compiler/all.T b/testsuite/tests/perf/compiler/all.T
--- a/testsuite/tests/perf/compiler/all.T
+++ b/testsuite/tests/perf/compiler/all.T
@@ -39,7 +39,7 @@
              # 2013-11-13 17 (x86/Windows, 64bit machine)
              # 2015-07-11 21 (x86/Linux, 64bit machine) use +RTS -G1
              # 2016-04-06 30 (x86/Linux, 64bit machine)
-           (wordsize(64), 83, 20)]),
+           (wordsize(64), 61, 20)]),
              #            28 (amd64/Linux)
              #            34 (amd64/Linux)
              # 2012-09-20 23 (amd64/Linux)
@@ -54,6 +54,7 @@
              # 2016-10-20 68, (amd64/Linux) allow top-level string literals
              #                See the comment 16 on #8472.
              # 2017-02-17 83  (amd64/Linux) Type-indexed Typeable
+             # 2017-03-31 61  (amd64/Linux) Fix memory leak in simplifier
       compiler_stats_num_field('max_bytes_used',
           [(platform('i386-unknown-mingw32'), 5719436, 20),
                                  # 2010-05-17 5717704 (x86/Windows)
@@ -70,7 +71,7 @@
              # 2016-04-06 9093608 (x86/Linux, 64bit machine)
              # 2017-03-24 9261052 (x86/Linux, 64-bit machine)
 
-           (wordsize(64), 19924328, 15)]),
+           (wordsize(64), 16679176, 15)]),
              # 2014-09-10 10463640, 10  # post-AMP-update (somewhat stabelish)
                # looks like the peak is around ~10M, but we're
                # unlikely to GC exactly on the peak.
@@ -84,6 +85,7 @@
              # 2016-10-12 17285216 (amd64/Linux) it's not entirely clear why
              # 2017-02-01 19924328 (amd64/Linux) Join points (#12988)
              # 2017-02-14 16393848 Early inline patch
+             # 2017-03-31 16679176 Fix memory leak in simplifier
 
       compiler_stats_num_field('bytes allocated',
           [(platform('i386-unknown-mingw32'), 301784492, 5),
@@ -325,7 +327,7 @@
             # 2016-04-06: 153261024 (x86/Linux) probably wildcard refactor
             # 2017-03-24: 134044092 (x86/Linux, 64-bit machine) Update
 
-           (wordsize(64), 259815560, 5)]),
+           (wordsize(64), 265950920, 5)]),
             # (amd64/Linux) (2011-06-28):  73259544
             # (amd64/Linux) (2013-02-07): 224798696
             # (amd64/Linux) (2013-08-02): 236404384, increase from roles
@@ -350,6 +352,7 @@
             #                                        of zonkTcType (Trac #11882)
             # (amd64/Darwin) (2017-01-23): 306222424 Presumably creep from recent changes (Typeable?)
             # (amd64/Linux) (2017-02-14): 259815560  Early inline patch: 9% improvement
+            # (amd64/Linux) (2017-03-31): 265950920  Fix memory leak in simplifier
 
 ###################################
 # deactivated for now, as this metric became too volatile recently
@@ -434,7 +437,7 @@
         # 2014-04-04:     346389856 (x86 Windows, 64 bit machine)
         # 2014-12-01:     390199244 (Windows laptop)
         # 2016-04-06:     570137436 (amd64/Linux) many reasons
-           (wordsize(64), 1065147968, 5)]),
+           (wordsize(64), 1037482512, 5)]),
         # expected value: 774595008 (amd64/Linux):
         # expected value: 735486328 (amd64/Linux) 2012/12/12:
         # expected value: 690742040 (amd64/Linux) Call Arity improvements
@@ -448,6 +451,7 @@
         # 2016-11-10:     1077429456 (amd64/Linux) Stop -dno-debug-output suppressing -ddump-tc-trace
         # 2017-02-17:     1517484488 (amd64/Linux) Type-indexed Typeable
         # 2017-03-03:     1065147968 (amd64/Linux) Share Typeable KindReps
+        # 2017-03-31:     1037482512 (amd64/Linux) Fix memory leak in simplifier
        only_ways(['normal'])
       ],
      compile,
@@ -713,7 +717,7 @@
            # Original:    381360728
            # 2014-07-31:  343005716 (Windows) (general round of updates)
            # 2017-03-24:  249904136 (x86/Linux, 64-bit machine)
-           (wordsize(64), 500707080, 10)])
+           (wordsize(64), 493596312, 10)])
            # prev:        795469104
            # 2014-07-17:  728263536 (general round of updates)
            # 2014-09-10:  785871680 post-AMP-cleanup
@@ -725,6 +729,7 @@
            # 2017-02-03:  764866144 Join points
            # 2017-02-14:  500707080 Early inline patch; 35% decrease!
            #                        Program size collapses in first simplification
+           # 2017-03-31:  493596312 Fix memory leak in simplifier
       ],
       compile,[''])