The survival of stage I melanoma patients was evaluated and compared with the detectable expression of HLA antigens. Of 904 patients who were surgically treated, 219 were HLA typed on peripheral blood lymphocytes. Four consecutive HLA typings were considered necessary. Median follow-up was 8 years. Two main groups of patients were considered: (a) patients with consistent detectable expression of antigens; and (b) patients with inconsistent detectable expression of antigens. Patients with consistent HLA antigens detection had an 8-year survival rate of 87.7% compared with 49.2% of patients with an inconsistent rate (P10(-7). Multivariate analysis of survival of the 182 HLA-typed patients who survived at least 24 months from surgery showed that two of the criteria had an independent impact on survival: tumour thickness (P 0.02) and HLA typing (P 2 x 10(-5). Inconsistent detection of HLA antigens on peripheral blood lymphocytes during the first 24 months after surgery is an indicator of poor prognosis in stage I melanoma patients.