diff --git a/src/c/main.d b/src/c/main.d
index cac637d83..9dbb8b350 100644
--- a/src/c/main.d
+++ b/src/c/main.d
@@ -170,7 +170,6 @@ _ecl_alloc_env(cl_env_ptr parent)
 #if defined(ECL_THREADS)
   output->bds_stack.tl_bindings_size = 0;
   output->bds_stack.tl_bindings = NULL;
-  output->cleanup = 0;
 #endif
   output->own_process = ECL_NIL;
   {
diff --git a/src/c/process.d b/src/c/process.d
index fc26b971b..aec804237 100644
--- a/src/c/process.d
+++ b/src/c/process.d
@@ -143,6 +143,28 @@ del_env(cl_env_ptr the_env)
   ecl_mutex_unlock(&ecl_core.processes_lock);
 }
 
+static void
+register_gc_thread()
+{
+#ifdef GBC_BOEHM
+  if (GC_thread_is_registered() == 0) {
+    struct GC_stack_base stack;
+    GC_get_stack_base(&stack);
+    GC_register_my_thread(&stack);
+  }
+#endif
+}
+
+static void
+unregister_gc_thread()
+{
+#ifdef GBC_BOEHM
+  if (GC_thread_is_registered() == 1) {
+    GC_unregister_my_thread();
+  }
+#endif
+}
+
 /* Run a process in the current system thread. */
 cl_env_ptr
 ecl_adopt_cpu()
@@ -151,28 +173,10 @@ ecl_adopt_cpu()
   struct ecl_interrupt_struct int_aux[1];
   cl_env_ptr the_env = ecl_process_env_unsafe();
   ecl_thread_t current;
-  int registered;
   if (the_env != NULL)
     return the_env;
   /* Ensure that the thread is known to the GC. */
-  /* FIXME this should be executed with hooks. */
-#ifdef GBC_BOEHM
-  {
-    struct GC_stack_base stack;
-    GC_get_stack_base(&stack);
-    switch (GC_register_my_thread(&stack)) {
-    case GC_SUCCESS:
-      registered = 1;
-      break;
-    case GC_DUPLICATE:
-      /* Thread was probably created using the GC hooks for thread creation. */
-      registered = 0;
-      break;
-    default:
-      ecl_internal_error("gc returned an impossible answer.");
-    }
-  }
-#endif
+  register_gc_thread();
   ecl_set_process_self(current);
   /* We need a fake env to allow for interrupts blocking and to set up frame
    * stacks or other stuff that is needed by ecl_init_env. Since the fake env is
@@ -186,7 +190,6 @@ ecl_adopt_cpu()
   env_aux->interrupt_struct->signal_queue = ECL_NIL;
   ecl_set_process_env(env_aux);
   env_aux->thread = current;
-  env_aux->cleanup = registered;
   ecl_init_env(env_aux);
 
   /* Allocate, initialize and switch to the real environment. */
@@ -202,11 +205,9 @@ ecl_adopt_cpu()
 void
 ecl_disown_cpu()
 {
-  int registered;
   cl_env_ptr the_env = ecl_process_env_unsafe();
   if (the_env == NULL)
     return;
-  registered = the_env->cleanup;
   ecl_disable_interrupts_env(the_env);
   /* FIXME this should be part of dealloc. */
   ecl_clear_bignum_registers(the_env);
@@ -216,10 +217,7 @@ ecl_disown_cpu()
   ecl_set_process_env(NULL);
   del_env(the_env);
   _ecl_dealloc_env(the_env);
-  /* FIXME thsi should be executed with hooks. */
-  if (registered) {
-    GC_unregister_my_thread();
-  }
+  unregister_gc_thread();
 }
 
 #ifdef ECL_WINDOWS_THREADS
diff --git a/src/h/external.h b/src/h/external.h
index e5bd86c04..9cebc9f4b 100755
--- a/src/h/external.h
+++ b/src/h/external.h
@@ -104,7 +104,6 @@ struct cl_env_struct {
         cl_object own_process; /* Backpointer to the host process. */
 #ifdef ECL_THREADS
         ecl_thread_t thread;
-        int cleanup;
 #endif
 
         /* -- System Interrupts ---------------------------------------------- */