A clinicopathologic study was done in 151 patients with malignant melanoma of the extremities who were surgically treated in Memorial Hospital and had 5-to 9-year followup. Microstaging was done according to the depth of invasion, as determined by Clark's levels as well as by direct measurement. This was related to treatment and to clinical course. There was a correlation between the depth of invasion by Clark's levels and the incidence of lymph node metastases in patients with Stage I melanoma who had elective node dissection. The incidence of nodal metastases was 4% for Level II, 7% FOR Level III, 25% for Level IV, and 70% for Level V. There was a correlation between Clark's level of invasion and survival aftter surgery. The 5-year cure rate was 100% for Level II, 88% for Level III, 60% for Level IV, and 15% for Level V melanoma. The presence of nodal metastases augured a much worse prognosis than Clark's level per se. In patients with Level IV melanoma, the 5-year cure rate was 82% in patients with negative nodes and 27% in those with nodal metastases after elective node dissection. Microstaging primary melanoma according to Clark's levels serves as a useful standard with which to compare surgical results. In this series of extremity melanomas there was no difference between local recurrence and lymphadenectomy for Level II melanoma. For Level III and Level IV melanoma, wide excision and lymphadenectomy gave higher cure rates than wide excision only, both at 5 and 9 years after surgery. The results were significant only for patients with Level III, however. Use of the measured depth of invasion added significant clinicopathologic information. The incidence of nodal metastases at elective node dissection was 5 to 9% for melanoma showing 0.6 to 2.0 mm of invasion, 22% for melanoma measuring 2.1 to 3.0 mm, and 39% for melanoma invading beyond 3.0 mm. The 5-year cure rate was 100% for melanoma measuring less than 1.0 mm, 83% for melanoma invading 1.1 to 2.0 mm, 58% for lesions measuring 2.1 to 3.0 mm, and 55% for melanoma invading over 3.0 mm. The microstage technique combining Clark's levels and the measured depth of invasion has an important use as a prognostic index and as a standard upon which to select treatment for primary melanoma of the extremities.