Microcystic adnexal carcinoma is an aggressive, locally destructive cutaneous neoplasm with a high rate of recurrence. This tumor is often misdiagnosed clinically and histologically. It usually occurs in middle-aged to older adults. We describe a 44-year-old man with a large microcystic adnexal carcinoma that was present for more than 20 years. The tumor invaded the perichondrium, muscle, nerve, and blood vessel adventitia. A review of the literature suggests that these tumors are often histologically misdiagnosed because the biopsy specimens may be too small to reveal all the characteristic histologic features. The clinical presence of marked induration, a smooth surface, and, possibly, sensory changes should alert the clinician to the possibility of this neoplasm. The initial biopsy specimen must be large enough to demonstrate the identifying histologic features. Mohs surgery is currently the treatment of choice for microcystic adnexal carcinoma, as it often spreads far beyond clinically evident tumor. (Arch Otolaryngol Head Neck Surg. 1996;122:179-182)