Scientific evidence for advisable excision margins for nonmelanotic skin carcinoma is poorly documented. Recommended excision margins vary from 2 to 15 mm. A prospective study was performed on 150 skin lesions excised over a 9-month period in an outpatient facility at the authors' institution. Primary nonmelanotic skin lesions were clinically diagnosed as either basal cell carcinoma (nodular, superficial, infiltrating, or sclerosing) or squamous cell carcinoma (well, moderately, or poorly differentiated). Macroscopic surgical excision margins were individually assessed, measured, and excised. Histopathologic analysis was then independently performed to determine the correct diagnosis and to measure the actual microscopic lateral and deep excision margins.Sixty-one percent of lesions were basal cell carcinoma, 25 percent were squamous cell carcinoma, and 15 percent were benign or premalignant. Diagnostic accuracy was 81 percent for basal cell and 59 percent for squamous cell carcinoma. The average diameter of the basal cell carcinoma was 12.1 mm; 47 percent of these lesions had a diameter of less than 10 mm. The average diameter of the squamous cell carcinoma was 16.9 mm; 26 percent of these lesions had a diameter of less than 10 mm. The mean surgical margin was 4.2 mm (3.2 mm adjusted for shrinkage), whereas the mean microscopic lateral margin was 3.4 mm. Overall, complete excision was achieved for 98 percent of basal cell carcinoma and 100 percent of squamous cell carcinoma. The raw data were analyzed to assess the suitability of 1-, 2-, 3-, or 4-mm surgical excision margins. A 4-mm surgical margin would give a microscopic lateral margin beyond one microscopic high-power field (0.5 mm) in 96 percent of cases of basal cell carcinoma and in 97 percent of cases of squamous cell carcinoma. The authors recommend a 4-mm surgical margin as the optimal treatment for skin lesions clinically diagnosed as basal cell or squamous cell carcinoma that are suitable for excision in an outpatient facility. Well-demarcated lesions, such as a nodular basal cell carcinoma, may be excised with a 3-mm margin.