We studied the clinical and histopathologic features of 165 basal cell carcinomas of the eyelids and classified the tumors into four types (nodular, ulcerative, morphea, and multicentric) that differ in incidence, results of incomplete excision, aggressiveness, and recurrence. The value of frozen-section monitoring of the surgical margins was evident. In 39 cases excised with monitoring of the surgical margins by frozen sections, there were no recurrences. In 126 cases treated by excisional biopsy without frozen-section monitoring, 34 (26.9%) were incompletely excised and seven (5.5%) recurred. The surgical treatment of basal cell carcinoma should always include the use of frozen-section monitoring of the surgical margins. This is especially important in the morphea, multicentric, and ulcerative types, in which the invasive extent of the tumor is often not apparent clinically. These forms are also more aggressive.