Although a thickness of less than or equal to 0.76 mm has been used to define biologically favorable (thin) melanoma, there is evidence that 1 mm may be a reasonable cutoff to categorize favorable extremity melanomas. This is tempered, however, by the claim that histologic regression in thin melanomas is associated with an increased metastatic rate. We have therefore addressed the following questions: Is 1 mm an appropriate cutoff point to define thin melanoma on the extremities? Does regression in a thin lesion truly signify a poor prognosis? Is the width of excision (narrow vs. wide) related to recurrence rates in these lesions? To address these issues we reviewed 48 patients with extremity melanomas, less than or equal to 1 mm in maximum thickness, treated at this institution during a 20-year period. Pathologic features included histologic type: superficial spreading (90%), nodular (6%), and not classified (4%); thickness: less than 0.76 mm (61%) and 0.76 to 1 mm (39%); and Clark's level: II (33%), III (63%), and IV (4%). A moderate or marked lymphoid infiltrate occurred in 75%, and histologic tumor regression was found in 50%. The median margin of excision, as stated by the surgeon, was 4 cm. The median margin measured by the pathologist in unfixed specimens was 3.5 cm. Although 13% had atypical melanocytic hyperplasia in the initial excisional biopsy margin, all reexcisions were clear. Of 21 patients having node dissections, none had nodal metastases. There were no recurrences or deaths due to melanoma (median follow-up: 90 months). We conclude that melanomas less than or equal to 1 mm in thickness on the extremities can be defined as biologically highly favorable, "thin" lesions. Foci of regression do not alter their behavior. Their favorable prognosis justifies conservative excision in most cases.