diff --git i/Lib/bdb.py w/Lib/bdb.py
index 59440a9..92994bf 100644
--- i/Lib/bdb.py
+++ w/Lib/bdb.py
@@ -25,6 +25,7 @@ class Bdb:
         self.breaks = {}
         self.fncache = {}
         self.frame_returning = None
+        self.botframe = None
 
     def canonic(self, filename):
         if filename == "<" + filename[1:-1] + ">":
@@ -36,15 +37,16 @@ class Bdb:
             self.fncache[filename] = canonic
         return canonic
 
-    def reset(self):
+    def reset(self, botframe=None):
         import linecache
         linecache.checkcache()
-        self.botframe = None
+        self.botframe = botframe
+        self.quitting = False
         self._set_stopinfo(None, None)
 
     def trace_dispatch(self, frame, event, arg):
-        if self.quitting:
-            return # None
+        if not self.botframe:
+            self.botframe = frame
         if event == 'line':
             return self.dispatch_line(frame)
         if event == 'call':
@@ -65,7 +67,9 @@ class Bdb:
     def dispatch_line(self, frame):
         if self.stop_here(frame) or self.break_here(frame):
             self.user_line(frame)
-            if self.quitting: raise BdbQuit
+            # Do not raise BdbQuit when debugging is started with set_trace
+            if self.quitting and self.botframe.f_back:
+                raise BdbQuit
         return self.trace_dispatch
 
     def dispatch_call(self, frame, arg):
@@ -78,7 +82,8 @@ class Bdb:
             # No need to trace this function
             return # None
         self.user_call(frame, arg)
-        if self.quitting: raise BdbQuit
+        if self.quitting and self.botframe.f_back:
+            raise BdbQuit
         return self.trace_dispatch
 
     def dispatch_return(self, frame, arg):
@@ -88,13 +93,15 @@ class Bdb:
                 self.user_return(frame, arg)
             finally:
                 self.frame_returning = None
-            if self.quitting: raise BdbQuit
+            if self.quitting and self.botframe.f_back:
+                raise BdbQuit
         return self.trace_dispatch
 
     def dispatch_exception(self, frame, arg):
         if self.stop_here(frame):
             self.user_exception(frame, arg)
-            if self.quitting: raise BdbQuit
+            if self.quitting and self.botframe.f_back:
+                raise BdbQuit
         return self.trace_dispatch
 
     # Normally derived classes don't override the following
@@ -214,9 +221,14 @@ class Bdb:
 
         If frame is not specified, debugging starts from caller's frame.
         """
+        # First disable tracing temporarily as set_trace() may be called while
+        # tracing is in use. For example when called from a signal handler and
+        # within a debugging session started with runcall().
+        sys.settrace(None)
+
         if frame is None:
             frame = sys._getframe().f_back
-        self.reset()
+        self.reset(botframe=self.botframe)
         while frame:
             frame.f_trace = self.trace_dispatch
             self.botframe = frame
@@ -230,10 +242,6 @@ class Bdb:
         if not self.breaks:
             # no breakpoints; run without debugger overhead
             sys.settrace(None)
-            frame = sys._getframe().f_back
-            while frame and frame is not self.botframe:
-                del frame.f_trace
-                frame = frame.f_back
 
     def set_quit(self):
         self.stopframe = self.botframe
@@ -401,7 +409,6 @@ class Bdb:
         except BdbQuit:
             pass
         finally:
-            self.quitting = 1
             sys.settrace(None)
 
     def runeval(self, expr, globals=None, locals=None):
@@ -419,7 +426,6 @@ class Bdb:
         except BdbQuit:
             pass
         finally:
-            self.quitting = 1
             sys.settrace(None)
 
     def runctx(self, cmd, globals, locals):
@@ -437,7 +443,6 @@ class Bdb:
         except BdbQuit:
             pass
         finally:
-            self.quitting = 1
             sys.settrace(None)
         return res
 
diff --git i/Lib/pdb.py w/Lib/pdb.py
index 5468d3f..51a6ea9 100755
--- i/Lib/pdb.py
+++ w/Lib/pdb.py
@@ -104,8 +104,8 @@ class Pdb(bdb.Bdb, cmd.Cmd):
         self.commands_bnum = None # The breakpoint number for which we are
                                   # defining a list
 
-    def reset(self):
-        bdb.Bdb.reset(self)
+    def reset(self, botframe=None):
+        bdb.Bdb.reset(self, botframe)
         self.forget()
 
     def forget(self):