diff --git a/swiftmpistepsim.c b/swiftmpistepsim.c
index 8a4a257098615243a3811d6250b93f657c938bd9..dc1573d8f20408d11d956cc5803457a931ccf3d8 100644
--- a/swiftmpistepsim.c
+++ b/swiftmpistepsim.c
@@ -38,6 +38,9 @@ static int verbose = 0;
 /* Attempt to keep original injection time differences. */
 static int usetics = 1;
 
+/* Set a data pattern and check we get this back, slow... */
+static int datacheck = 0;
+
 /* Integer types of send and recv tasks, must match log. */
 static const int task_type_send = 22;
 static const int task_type_recv = 23;
@@ -64,6 +67,35 @@ static int volatile todo_send = 0;
 // XXX need to store this in the data file.
 static double log_clocks_cpufreq = 2194844448.0;
 
+/**
+ * @brief fill a data area with a pattern that can be checked for changes.
+ *
+ * @param size size of data in bytes.
+ * @param data the data to fill.
+ */
+static void datacheck_fill(size_t size, void *data) {
+  unsigned char *p = (unsigned char *)data;
+  for (size_t i = 0; i < size; i++) {
+      p[i] = 170; /* 10101010 in bits. */
+  }
+}
+
+/**
+ * @brief test a filled data area for our pattern.
+ *
+ * @param size size of data in bytes.
+ * @param data the data to fill.
+ *
+ * @result 1 on success, 0 otherwise.
+ */
+static int datacheck_test(size_t size, void *data) {
+  unsigned char *p = (unsigned char *)data;
+  for (size_t i = 0; i < size; i++) {
+    if (p[i] != 170) return 0;
+  }
+  return 1;
+}
+
 /**
  * @brief Injection thread, initiates MPI_Isend and MPI_Irecv requests.
  *
@@ -128,6 +160,11 @@ static void *inject_thread(void *arg) {
     int err = 0;
     if (log->type == task_type_send) {
       log->data = calloc(log->size, 1);
+
+      /* Fill data with pattern. */
+      if (datacheck) datacheck_fill(log->size, log->data);
+
+      /* And send. */
       err = MPI_Isend(log->data, log->size, MPI_BYTE, log->otherrank, log->tag,
                       subtypeMPI_comms[log->subtype], &log->req);
 
@@ -137,6 +174,8 @@ static void *inject_thread(void *arg) {
       atomic_inc(&todo_send);
 
     } else {
+
+      /* Ready to receive. */
       log->data = calloc(log->size, 1);
       err = MPI_Irecv(log->data, log->size, MPI_BYTE, log->otherrank, log->tag,
                       subtypeMPI_comms[log->subtype], &log->req);
@@ -229,6 +268,12 @@ static void queue_runner(struct mpiuse_log_entry **logs, int volatile *nr_logs,
         if (dt > lmaxt) lmaxt = dt;
 
         if (res) {
+          /* Check data sent data is unchanged and received data is as
+           * expected. */
+          if (datacheck && !datacheck_test(log->size, log->data)) {
+            error("Data mismatch on completion");
+          }
+
           /* Done, clean up. */
           log->done = 1;
           log->endtic = getticks();
@@ -392,8 +437,11 @@ int main(int argc, char *argv[]) {
   /* Handle the command-line, we expect a mpiuse data file to read and various
    * options. */
   int opt;
-  while ((opt = getopt(argc, argv, "vf")) != -1) {
+  while ((opt = getopt(argc, argv, "vfd")) != -1) {
     switch (opt) {
+      case 'd':
+        datacheck = 1;
+        break;
       case 'f':
         usetics = 0;
         break;
@@ -438,6 +486,10 @@ int main(int argc, char *argv[]) {
   if (myrank == 0) {
     message("Start of MPI tests");
     message("==================");
+    if (verbose) {
+      if (usetics) message("using fast untimed injections");
+      if (datacheck) message("checking data pattern on send and recv completion");
+    }
   }
 
   /* Make three threads, one for injecting tasks and two to check for