Biopsy specimens from 12 patients with metastatic melanoma were longitudinally analysed to evaluate changes in proliferation activity and CD4+/CD8+ ratios during the course of the disease. The primary tumours of the patients who subsequently had metastatic disease were also each matched with tumours from two controls whose disease remained localized, and were compared with regard to tumour proliferation. Immunohistochemistry was performed using the avidin-biotin complex (ABC) immunoperoxidase technique, using bcl-2, p53, mdm-2 and Ki-67 as the primary monoclonal antibodies, and the percentage of positively stained melanocytic cells was calculated. Frozen sections were also available from metastatic lesions excised from eight of our patients before treatment initiation and at the time of disease progression. These specimens were prepared for microscopy, and quantitative characterization of CD4+ (OKT 4a) and CD8+ (OKT 8) cells was performed. Compared with the localized melanomas bcl-2 expression was higher in those primary melanomas that later metastasized (P = 0.068, Wilcoxon; P = 0.038, median test). Mdm-2 and Ki-67 expression did not differ in the primary tumours of patients and controls, but a statistically significant trend was observed towards increasing expression with the progression of the disease (two-sided exact P-values: 0.04 and 0.05, respectively). Patients with a low Ki-67 index in their first metastasis had a better prognosis when compared with patients with high indexes (P = 0.008, log-rank). Furthermore, most patients with decreasing CD4+/CD8+ ratios had increasing p53 immunoreactivity. Our findings suggest that Ki-67 and bcl-2 may be useful for predicting the prognosis of melanoma patients. Mdm-2 is a new but promising marker in melanoma and deserves further evaluation.