The value of prophylactic node dissection was studied in 147 patients with nonsuperficial malignant melanoma of cutaneous origin; all had clinical stage I disease. Seventy-three patients had prophylactic node dissection and 74 did not. Survival rates were calculated by the actuarial method and were age and sex adjusted. Five-year crude survival rates for these two groups were 62 and 29%, respectively, and the adjusted rates were 70 and 33%, respectively. These significant differences (P less than 0.001) were maintained at 10 years. The difference in survival in the two groups cannot be explained on the basis of age, sex, year of operation, size or location of the primary tumour, or previous incisional biopsy. It is concluded that prophylactic node dissection contributed appreciably to increased survival in this study.