Our spelling correction program is part of a system for understanding illformed input in an intelligent tutoring system for medical students. Speed and user-friendliness were the most important considerations in the design. The system can correct most kinds of spelling errors including order reversal, missing characters, added characters, and character substitutions. It also handles novel abbreviations and word boundary errors. It is implemented on a Xerox 1108 AI machine in Interlisp-D. The lexicon is stored in a trie structure to speed up searching. We provide the students with a full-screen editor and an input tracer to simplify the input process as much as possible.