diff --git a/doc/sys_arch.txt b/doc/sys_arch.txt
index b517de26..44fa3372 100644
--- a/doc/sys_arch.txt
+++ b/doc/sys_arch.txt
@@ -120,10 +120,10 @@ If threads are supported by the underlying operating system and if
 such functionality is needed in lwIP, the following function will have
 to be implemented as well:
 
-- sys_thread_t sys_thread_new(void (* thread)(void *arg), void *arg)
+- sys_thread_t sys_thread_new(void (* thread)(void *arg), void *arg, int prio)
 
-  Starts a new thread that will begin its execution in the function
-  "thread()". The "arg" argument will be passed as an argument to the
+  Starts a new thread with priority "prio" that will begin its execution in the
+  function "thread()". The "arg" argument will be passed as an argument to the
   thread() function. The id of the new thread is returned.
 
 - sys_prot_t sys_arch_protect(void)
diff --git a/src/api/tcpip.c b/src/api/tcpip.c
index 77dc2145..cfeeb9e7 100644
--- a/src/api/tcpip.c
+++ b/src/api/tcpip.c
@@ -165,7 +165,7 @@ tcpip_init(void (* initfunc)(void *), void *arg)
   tcpip_init_done = initfunc;
   tcpip_init_done_arg = arg;
   mbox = sys_mbox_new();
-  sys_thread_new(tcpip_thread, NULL);
+  sys_thread_new(tcpip_thread, NULL, TCPIP_THREAD_PRIO);
 }
 /*-----------------------------------------------------------------------------------*/
 
diff --git a/src/include/lwip/opt.h b/src/include/lwip/opt.h
index 347d1a46..9f0660bd 100644
--- a/src/include/lwip/opt.h
+++ b/src/include/lwip/opt.h
@@ -296,6 +296,18 @@ a lot of data that needs to be copied, this should be set high. */
 #define LWIP_COMPAT_SOCKETS     1
 #endif
 
+
+#ifndef TCPIP_THREAD_PRIO
+#define TCPIP_THREAD_PRIO 1
+#endif
+
+#ifndef SLIPIF_THREAD_PRIO
+#define SLIPIF_THREAD_PRIO 1
+#endif
+
+#ifndef DEFAULT_THREAD_PRIO
+#define DEFAULT_THREAD_PRIO 1
+#endif
 /* ---------- Statistics options ---------- */
 #ifndef LWIP_STATS
 #define LWIP_STATS	1
diff --git a/src/include/lwip/sys.h b/src/include/lwip/sys.h
index 8c05cf42..94bc0790 100644
--- a/src/include/lwip/sys.h
+++ b/src/include/lwip/sys.h
@@ -57,7 +57,7 @@ struct sys_timeout {u8_t dummy;};
 #define sys_mbox_post(m,d)
 #define sys_mbox_free(m)
 
-#define sys_thread_new(t,a)
+#define sys_thread_new(t,a,p)
 
 /* We don't need protection if there is no OS */
 #define SYS_ARCH_DECL_PROTECT(lev)
@@ -157,7 +157,7 @@ void sys_arch_unprotect(sys_prot_t pval);
 #endif /* SYS_ARCH_PROTECT */
 
 /* Thread functions. */
-sys_thread_t sys_thread_new(void (* thread)(void *arg), void *arg);
+sys_thread_t sys_thread_new(void (* thread)(void *arg), void *arg, int prio);
 
 /* The following functions are used only in Unix code, and
    can be omitted when porting the stack. */
diff --git a/src/netif/slipif.c b/src/netif/slipif.c
index 2375db44..ef43a04d 100644
--- a/src/netif/slipif.c
+++ b/src/netif/slipif.c
@@ -212,6 +212,6 @@ slipif_init(struct netif *netif)
   if (!netif->state)
       return ERR_IF;
 
-  sys_thread_new(slipif_loop, netif);
+  sys_thread_new(slipif_loop, netif, SLIPIF_THREAD_PRIO);
   return ERR_OK;
 }