I use McCLIM with mit-clx. When I run any demo application I always get the same connection error to my local X server. The problem, I think, is in erroneous fetching of protocol name from ~/.Xautority. I remember the same bug in telent-clx some time ago. I made the same changes as in telent-clx against clisp/modules/clx/mit-clx/display.lisp . Now McCLIM application runs well.