The authors have studied the natural history of 377 patients with Stage I cutaneous malignant melanoma followed at the Arizona Cancer Center, Tucson. Two hundred eight patients, or 55%, remained free of metastatic disease after a median follow-up of 30 months. The survival at 5, 8, and 10 years was 69, 65, and 63%, respectively. Natural breakpoints in Breslow thickness for survival occurred at 0.85, 1.95, and 4.00 mm. These are not significantly different from those found by other investigators. A step-down multivariate analysis using the Cox regression model yielded four factors as highly significant in predicting survival: Breslow thickness (P less than 0.001), an age/sex interaction (P = 0.0012), clinical ulceration (P = 0.0039), and a prophylactic node dissection (P = 0.019). No predictive value for a BANS or non-BANS location was detected. These results are discussed in reference to other large series which describe the natural history of cutaneous melanoma.