diff --git a/Makefile.in b/Makefile.in
index 16fe30fd..868bf8cc 100644
--- a/Makefile.in
+++ b/Makefile.in
@@ -189,6 +189,7 @@ TESTS_UNIT := \
 	$(srcroot)test/unit/ql.c \
 	$(srcroot)test/unit/qr.c \
 	$(srcroot)test/unit/rb.c \
+	$(srcroot)test/unit/retained.c \
 	$(srcroot)test/unit/rtree.c \
 	$(srcroot)test/unit/SFMT.c \
 	$(srcroot)test/unit/size_classes.c \
diff --git a/test/unit/retained.c b/test/unit/retained.c
new file mode 100644
index 00000000..883bf4af
--- /dev/null
+++ b/test/unit/retained.c
@@ -0,0 +1,179 @@
+#include "test/jemalloc_test.h"
+
+static unsigned		arena_ind;
+static size_t		sz;
+static size_t		esz;
+#define NEPOCHS		8
+#define PER_THD_NALLOCS	1
+static atomic_u_t	epoch;
+static atomic_u_t	nfinished;
+
+static unsigned
+do_arena_create(extent_hooks_t *h) {
+	unsigned arena_ind;
+	size_t sz = sizeof(unsigned);
+	assert_d_eq(mallctl("arenas.create", (void *)&arena_ind, &sz,
+	    (void *)(h != NULL ? &h : NULL), (h != NULL ? sizeof(h) : 0)), 0,
+	    "Unexpected mallctl() failure");
+	return arena_ind;
+}
+
+static void
+do_arena_destroy(unsigned arena_ind) {
+	size_t mib[3];
+	size_t miblen;
+
+	miblen = sizeof(mib)/sizeof(size_t);
+	assert_d_eq(mallctlnametomib("arena.0.destroy", mib, &miblen), 0,
+	    "Unexpected mallctlnametomib() failure");
+	mib[1] = (size_t)arena_ind;
+	assert_d_eq(mallctlbymib(mib, miblen, NULL, NULL, NULL, 0), 0,
+	    "Unexpected mallctlbymib() failure");
+}
+
+static void
+do_refresh(void) {
+	uint64_t epoch = 1;
+	assert_d_eq(mallctl("epoch", NULL, NULL, (void *)&epoch,
+	    sizeof(epoch)), 0, "Unexpected mallctl() failure");
+}
+
+static size_t
+do_get_size_impl(const char *cmd, unsigned arena_ind) {
+	size_t mib[4];
+	size_t miblen = sizeof(mib) / sizeof(size_t);
+	size_t z = sizeof(size_t);
+
+	assert_d_eq(mallctlnametomib(cmd, mib, &miblen),
+	    0, "Unexpected mallctlnametomib(\"%s\", ...) failure", cmd);
+	mib[2] = arena_ind;
+	size_t size;
+	assert_d_eq(mallctlbymib(mib, miblen, (void *)&size, &z, NULL, 0),
+	    0, "Unexpected mallctlbymib([\"%s\"], ...) failure", cmd);
+
+	return size;
+}
+
+static size_t
+do_get_active(unsigned arena_ind) {
+	return do_get_size_impl("stats.arenas.0.pactive", arena_ind) * PAGE;
+}
+
+static size_t
+do_get_mapped(unsigned arena_ind) {
+	return do_get_size_impl("stats.arenas.0.mapped", arena_ind);
+}
+
+static void *
+thd_start(void *arg) {
+	for (unsigned next_epoch = 1; next_epoch < NEPOCHS; next_epoch++) {
+		/* Busy-wait for next epoch. */
+		unsigned cur_epoch;
+		spin_t spinner = SPIN_INITIALIZER;
+		while ((cur_epoch = atomic_load_u(&epoch, ATOMIC_ACQUIRE)) !=
+		    next_epoch) {
+			spin_adaptive(&spinner);
+		}
+		assert_u_eq(cur_epoch, next_epoch, "Unexpected epoch");
+
+		/*
+		 * Allocate.  The main thread will reset the arena, so there's
+		 * no need to deallocate.
+		 */
+		for (unsigned i = 0; i < PER_THD_NALLOCS; i++) {
+			void *p = mallocx(sz, MALLOCX_ARENA(arena_ind) |
+			    MALLOCX_TCACHE_NONE
+			    );
+			assert_ptr_not_null(p,
+			    "Unexpected mallocx() failure\n");
+		}
+
+		/* Let the main thread know we've finished this iteration. */
+		atomic_fetch_add_u(&nfinished, 1, ATOMIC_RELEASE);
+	}
+
+	return NULL;
+}
+
+TEST_BEGIN(test_retained) {
+	test_skip_if(!config_stats);
+
+	arena_ind = do_arena_create(NULL);
+	sz = nallocx(HUGEPAGE, 0);
+	esz = sz + large_pad;
+
+	atomic_store_u(&epoch, 0, ATOMIC_RELAXED);
+
+	unsigned nthreads = ncpus * 2;
+	VARIABLE_ARRAY(thd_t, threads, nthreads);
+	for (unsigned i = 0; i < nthreads; i++) {
+		thd_create(&threads[i], thd_start, NULL);
+	}
+
+	for (unsigned e = 1; e < NEPOCHS; e++) {
+		atomic_store_u(&nfinished, 0, ATOMIC_RELEASE);
+		atomic_store_u(&epoch, e, ATOMIC_RELEASE);
+
+		/* Wait for threads to finish allocating. */
+		spin_t spinner = SPIN_INITIALIZER;
+		while (atomic_load_u(&nfinished, ATOMIC_ACQUIRE) < nthreads) {
+			spin_adaptive(&spinner);
+		}
+
+		/*
+		 * Assert that retained is no more than the sum of size classes
+		 * that should have been used to satisfy the worker threads'
+		 * requests, discounting per growth fragmentation.
+		 */
+		do_refresh();
+
+		size_t allocated = esz * nthreads * PER_THD_NALLOCS;
+		size_t active = do_get_active(arena_ind);
+		assert_zu_le(allocated, active, "Unexpected active memory");
+		size_t mapped = do_get_mapped(arena_ind);
+		assert_zu_le(active, mapped, "Unexpected mapped memory");
+
+		arena_t *arena = arena_get(tsdn_fetch(), arena_ind, false);
+		size_t usable = 0;
+		size_t fragmented = 0;
+		for (pszind_t pind = psz2ind(HUGEPAGE); pind <
+		    arena->extent_grow_next; pind++) {
+			size_t psz = pind2sz(pind);
+			size_t psz_fragmented = psz % esz;
+			size_t psz_usable = psz - psz_fragmented;
+			/*
+			 * Only consider size classes that wouldn't be skipped.
+			 */
+			if (psz_usable > 0) {
+				assert_zu_lt(usable, allocated,
+				    "Excessive retained memory "
+				    "(%#zx[+%#zx] > %#zx)", usable, psz_usable,
+				    allocated);
+				fragmented += psz_fragmented;
+				usable += psz_usable;
+			}
+		}
+
+		/*
+		 * Clean up arena.  Destroying and recreating the arena
+		 * is simpler that specifying extent hooks that deallocate
+		 * (rather than retaining) during reset.
+		 */
+		do_arena_destroy(arena_ind);
+		assert_u_eq(do_arena_create(NULL), arena_ind,
+		    "Unexpected arena index");
+	}
+
+	for (unsigned i = 0; i < nthreads; i++) {
+		thd_join(threads[i], NULL);
+	}
+
+	do_arena_destroy(arena_ind);
+}
+TEST_END
+
+int
+main(void) {
+	return test(
+	    test_retained);
+}