Although wide surgical excision is the accepted treatment for thin malignant melanomas, there is reason to believe that narrower margins may be adequate. We conducted a randomized prospective study to assess the efficacy of narrow excision (excision with 1-cm margins) for primary melanomas no thicker than 2 mm. Narrow excision was performed in 305 patients, and wide excision (margins of 3 cm or more) was performed in 307 patients. The major prognostic criteria were well balanced in the two groups. The mean thickness of melanomas was 0.99 mm in the narrow-excision group and 1.02 mm in the wide-excision group. The subsequent development of metastatic disease involving regional nodes and distant organs was not different in the two groups (4.6 and 2.3 percent, respectively, in the narrow-excision group, as compared with 6.5 and 2.6 percent in the wide-excision group). Disease-free survival rates and overall survival rates (mean follow-up period, 55 months) were also similar in the two groups. Only three patients had a local recurrence as a first relapse. All had undergone narrow excision, and each had a primary melanoma with a thickness of 1 mm or more. The absence of local recurrence in the group of patients with a primary melanoma thinner than 1 mm and the very low rate of local recurrences indicate that narrow excision is a safe and effective procedure for such patients. (N Engl J Med 1988; 318:1159–62.) THE question of how much surrounding normal skin should be removed during the excision of primary melanomas of the skin has never been properly answered. For decades, wide excision (with margins of 3 to 5 cm) has been universally accepted as the treatment of choice. In 1977, however, Breslow and Macht1 reported that narrow resection margins may be satisfactory in the treatment of very thin melanomas. Subsequent reports234567891011 have also supported the conservative surgical approach to local control of the primary tumor. Nevertheless, there are several points of disagreement, including how thick a primary melanoma can be and still be. © 1988, Massachusetts Medical Society. All rights reserved.