A material consisting of 82 stage I cutaneous melanomas was analysed clinically, histopathologically and karyometrically. High age, deep Clark level of invasion and thick melanomas were associated with a negative prognosis. There was a good correlation between the nuclear area and the melanoma thickness. By applying Cox's proportional hazard method to clinical, histopathological and karyometric data, it was demonstrated that karyometry provided both prognostic and biological information. Both a large variation (percentile 90-percentile 50) in nuclear area and a large standard deviation of form factor was related to a negative prognosis (p less than 0.0003 and p less than 0.04 respectively).