Basal cell carcinomas frequently extend beyond their visible borders. Therefore, the goal of surgical therapy must be to eradicate both the clinically apparent tumor and its microscopic extension into the surrounding normal-appearing skin. This entails excising the tumor along with a margin of clinically normal skin. Unfortunately, there is no agreement as to the optimal width of surgical margins. We therefore studied 117 cases of previously untreated, well-demarcated basal cell carcinoma. Prior to excision, the normal-appearing skin surrounding the tumor was marked in 2-mm increments. The tumor was then excised using Mohs micrographic surgery. The extent of the subclinical tumor invasion was calculated from the presurgical skin markings. For tumors with a diameter less than 2 cm, a minimum margin of 4 mm was necessary to totally eradicate the tumor in more than 95% of cases.