Background: Incomplete Mohs micrographic surgery (MMS) is the cessation of MMS while the tumor margins are known to be still positive. Objective: Our purpose was to examine the factors behind unplanned incomplete MMS and to identify means of avoiding and managing this situation when it arises. Methods: We performed a retrospective case review examining clinical presentations, histologic features, and management of incomplete MMS. Results: Fifteen of 10,346 procedures (0.15%) were identified as incomplete MMS. Complete records were available in 14 cases. The age range was 30 to 90 years; the study comprised 10 men and 4 women. The tumors included 9 basal cell and 4 squamous cell carcinomas and 1 dermatofibrosarcoma protuberans. The sites involved were nose, medial canthus, ear, scalp, and lower eyelid. Twelve cases dealt with unresectable disease, whereas two patients were unable to tolerate further surgery. Of the unresectable cases, MMS was terminated because of ongoing multifocal positive skin margins, bony invasion, or extension of tumor to other locations. Surgical defects were repaired, whereas residual disease was managed with a variety of methods. Conclusion: Incomplete MMS is a rare problem of either unresectable disease or inability of the patient to tolerate the procedure. Preoperative planning may help to identify both subgroups of patients. A multispecialty approach to managing these cases is often necessary. (J Am Acad Dermatol 2000;42:814-9.)