diff --git a/sys/kern/syscalls.master b/sys/kern/syscalls.master
index 705bc9dac800c..7551bd45f8396 100644
--- a/sys/kern/syscalls.master
+++ b/sys/kern/syscalls.master
@@ -111,6 +111,7 @@
 ; limited set of output files.  Before the first syscalls, #include lines will
 ; be copied and %%ABI_HEADERS%% expanded.  Between system call entries,
 ; all lines beginning with # will be copied.  Caveat Emptor.
+; WARNING: this functionality is deprecated.
 
 #include <sys/param.h>
 #include <sys/sysent.h>
diff --git a/sys/tools/syscalls/core/freebsd-syscall.lua b/sys/tools/syscalls/core/freebsd-syscall.lua
index 1f82dd414ea0b..49b2781a6a39d 100644
--- a/sys/tools/syscalls/core/freebsd-syscall.lua
+++ b/sys/tools/syscalls/core/freebsd-syscall.lua
@@ -50,6 +50,7 @@ function FreeBSDSyscall:parseSysfile()
 	local incs = ""
 	local prolog = ""
 	local first = true
+	local cpp_warned = false
 	local s
 	for line in fh:lines() do
 		line = line:gsub(commentExpr, "") -- Strip any comments.
@@ -82,6 +83,11 @@ function FreeBSDSyscall:parseSysfile()
 				incs = incs .. h .. "\n"
 			end
 		elseif line:match("^#") then
+			if not cpp_warned then
+				util.warn("use of non-include cpp " ..
+				    "directives is deprecated")
+				cpp_warned = true
+			end
 			prolog = prolog .. line .. "\n"
 		else
 			s = syscall:new()