diff --git a/dat/themerms.lua b/dat/themerms.lua
index 85c3e18f4..db831a496 100644
--- a/dat/themerms.lua
+++ b/dat/themerms.lua
@@ -1,7 +1,10 @@
 
 -- themerooms is an array of tables and/or functions.
--- the tables define "frequency" and "contents",
--- a plain function has frequency of 1
+-- the tables define "frequency", "contents", "mindiff" and "maxdiff".
+-- frequency is optional; if omitted, 1 is assumed.
+-- mindiff and maxdiff are optional and independent; if omitted, the room is
+-- not constrained by level difficulty.
+-- a plain function has frequency of 1, and no difficulty constraints.
 -- des.room({ type = "ordinary", filled = 1 })
 --   - ordinary rooms can be converted to shops or any other special rooms.
 --   - filled = 1 means the room gets random room contents, even if it
@@ -538,36 +541,45 @@ end });
 
 };
 
-local total_frequency = 0;
-for i = 1, #themerooms do
-   local t = type(themerooms[i]);
+function is_eligible(room)
+   local t = type(room);
+   local diff = nh.level_difficulty();
    if (t == "table") then
-      total_frequency = total_frequency + themerooms[i].frequency;
+      if (room.mindiff ~= nil and diff < room.mindiff) then
+         return false
+      elseif (room.maxdiff ~= nil and diff > room.maxdiff) then
+         return false
+      end
    elseif (t == "function") then
-      total_frequency = total_frequency + 1;
+      -- functions currently have no constraints
    end
-end
-
-if (total_frequency == 0) then
-   error("Theme rooms total_frequency == 0");
+   return true
 end
 
 function themerooms_generate()
-   local pick = nh.rn2(total_frequency);
+   local pick = 1;
+   local total_frequency = 0;
    for i = 1, #themerooms do
-      local t = type(themerooms[i]);
-      if (t == "table") then
-         pick = pick - themerooms[i].frequency;
-         if (pick < 0) then
-            themerooms[i].contents();
-            return;
+      -- Reservoir sampling: select one room from the set of eligible rooms,
+      -- which may change on different levels because of level difficulty.
+      if is_eligible(themerooms[i]) then
+         local this_frequency;
+         if (type(themerooms[i]) == "table" and themerooms[i].frequency ~= nil) then
+            this_frequency = themerooms[i].frequency;
+         else
+            this_frequency = 1;
          end
-      elseif (t == "function") then
-         pick = pick - 1;
-         if (pick < 0) then
-            themerooms[i]();
-            return;
+         total_frequency = total_frequency + this_frequency;
+         if (nh.rn2(total_frequency) < this_frequency) then
+            pick = i;
          end
       end
    end
+
+   local t = type(themerooms[pick]);
+   if (t == "table") then
+      themerooms[pick].contents();
+   elseif (t == "function") then
+      themerooms[pick]();
+   end
 end
diff --git a/src/nhlua.c b/src/nhlua.c
index 24bcc5d59..b3906592b 100644
--- a/src/nhlua.c
+++ b/src/nhlua.c
@@ -34,6 +34,7 @@ static int FDECL(nhl_ing_suffix, (lua_State *));
 static int FDECL(nhl_an, (lua_State *));
 static int FDECL(nhl_rn2, (lua_State *));
 static int FDECL(nhl_random, (lua_State *));
+static int FDECL(nhl_level_difficulty, (lua_State *));
 static void FDECL(init_nhc_data, (lua_State *));
 static int FDECL(nhl_push_anything, (lua_State *, int, void *));
 static int FDECL(nhl_meta_u_index, (lua_State *));
@@ -657,6 +658,21 @@ lua_State *L;
     return 1;
 }
 
+/* level_difficulty() */
+static int
+nhl_level_difficulty(L)
+lua_State *L;
+{
+    int argc = lua_gettop(L);
+    if (argc == 0) {
+        lua_pushinteger(L, level_difficulty());
+    }
+    else {
+        nhl_error(L, "level_difficulty should not have any args");
+    }
+    return 1;
+}
+
 /* get mandatory integer value from table */
 int
 get_table_int(L, name)
@@ -831,6 +847,7 @@ static const struct luaL_Reg nhl_functions[] = {
     {"an", nhl_an},
     {"rn2", nhl_rn2},
     {"random", nhl_random},
+    {"level_difficulty", nhl_level_difficulty},
     {NULL, NULL}
 };