diff --git a/builtin-read-tree.c b/builtin-read-tree.c
index 2a3a32cbfe..0c1f40d10d 100644
--- a/builtin-read-tree.c
+++ b/builtin-read-tree.c
@@ -64,6 +64,34 @@ static int exclude_per_directory_cb(const struct option *opt, const char *arg,
 	return 0;
 }
 
+static void debug_stage(const char *label, struct cache_entry *ce,
+			struct unpack_trees_options *o)
+{
+	printf("%s ", label);
+	if (!ce)
+		printf("(missing)\n");
+	else if (ce == o->df_conflict_entry)
+		printf("(conflict)\n");
+	else
+		printf("%06o #%d %s %.8s\n",
+		       ce->ce_mode, ce_stage(ce), ce->name,
+		       sha1_to_hex(ce->sha1));
+}
+
+static int debug_merge(struct cache_entry **stages, struct unpack_trees_options *o)
+{
+	int i;
+
+	printf("* %d-way merge\n", o->merge_size);
+	debug_stage("index", stages[0], o);
+	for (i = 1; i <= o->merge_size; i++) {
+		char buf[24];
+		sprintf(buf, "ent#%d", i);
+		debug_stage(buf, stages[i], o);
+	}
+	return 0;
+}
+
 static struct lock_file lock_file;
 
 int cmd_read_tree(int argc, const char **argv, const char *unused_prefix)
@@ -98,6 +126,8 @@ int cmd_read_tree(int argc, const char **argv, const char *unused_prefix)
 		  PARSE_OPT_NONEG, exclude_per_directory_cb },
 		OPT_SET_INT('i', NULL, &opts.index_only,
 			    "don't check the working tree after merging", 1),
+		OPT_SET_INT(0, "debug-unpack", &opts.debug_unpack,
+			    "debug unpack-trees", 1),
 		OPT_END()
 	};
 
@@ -165,6 +195,9 @@ int cmd_read_tree(int argc, const char **argv, const char *unused_prefix)
 			opts.head_idx = 1;
 	}
 
+	if (opts.debug_unpack)
+		opts.fn = debug_merge;
+
 	cache_tree_free(&active_cache_tree);
 	for (i = 0; i < nr_trees; i++) {
 		struct tree *tree = trees[i];
@@ -174,6 +207,9 @@ int cmd_read_tree(int argc, const char **argv, const char *unused_prefix)
 	if (unpack_trees(nr_trees, t, &opts))
 		return 128;
 
+	if (opts.debug_unpack)
+		return 0; /* do not write the index out */
+
 	/*
 	 * When reading only one tree (either the most basic form,
 	 * "-m ent" or "--reset ent" form), we can obtain a fully
diff --git a/unpack-trees.c b/unpack-trees.c
index 74cabc36ff..10e8871a74 100644
--- a/unpack-trees.c
+++ b/unpack-trees.c
@@ -501,6 +501,38 @@ static struct cache_entry *find_cache_entry(struct traverse_info *info,
 		return NULL;
 }
 
+static void debug_path(struct traverse_info *info)
+{
+	if (info->prev) {
+		debug_path(info->prev);
+		if (*info->prev->name.path)
+			putchar('/');
+	}
+	printf("%s", info->name.path);
+}
+
+static void debug_name_entry(int i, struct name_entry *n)
+{
+	printf("ent#%d %06o %s\n", i,
+	       n->path ? n->mode : 0,
+	       n->path ? n->path : "(missing)");
+}
+
+static void debug_unpack_callback(int n,
+				  unsigned long mask,
+				  unsigned long dirmask,
+				  struct name_entry *names,
+				  struct traverse_info *info)
+{
+	int i;
+	printf("* unpack mask %lu, dirmask %lu, cnt %d ",
+	       mask, dirmask, n);
+	debug_path(info);
+	putchar('\n');
+	for (i = 0; i < n; i++)
+		debug_name_entry(i, names + i);
+}
+
 static int unpack_callback(int n, unsigned long mask, unsigned long dirmask, struct name_entry *names, struct traverse_info *info)
 {
 	struct cache_entry *src[MAX_UNPACK_TREES + 1] = { NULL, };
@@ -511,6 +543,9 @@ static int unpack_callback(int n, unsigned long mask, unsigned long dirmask, str
 	while (!p->mode)
 		p++;
 
+	if (o->debug_unpack)
+		debug_unpack_callback(n, mask, dirmask, names, info);
+
 	/* Are we supposed to look at the index too? */
 	if (o->merge) {
 		while (1) {
diff --git a/unpack-trees.h b/unpack-trees.h
index 9a0733ea85..701dca59a8 100644
--- a/unpack-trees.h
+++ b/unpack-trees.h
@@ -28,6 +28,7 @@ struct unpack_trees_options {
 		     skip_unmerged,
 		     initial_checkout,
 		     diff_index_cached,
+		     debug_unpack,
 		     gently;
 	const char *prefix;
 	int cache_bottom;