In various tumors, proliferative activity has been demonstrated to be closely related to the degree of malignancy. In the present study, we examined the proliferative pool in a total of 25 melanocytic (nevocytic) skin tumors by immunohistochemical staining with Ki 67 monoclonal antibody (labeling the G1, S, G2-phase of the cell cycle) and quantitative morphological evaluation. There were highly significant differences in the numerical density of Ki 67-positive cells between nevocellular nevi (number of positive cells: 2.2 +/- 0.7 X 10(3)/mm3), primary malignant melanoma (6.3 +/- 1.9 X 10(3)/mm3), and metastatic malignant melanoma (47.0 +/- 9.2 X 10(3)/mm3). Within the malignant melanoma group, there was a significant correlation between proliferative activity and maximum tumor thickness (r = 0.85; p less than 0.05). Comparison with previous studies using tritiated thymidine labeling and colony-forming efficiency shows that the stereological evaluation of Ki 67 staining in melanocytic skin tumors provides a reliable estimate of proliferative activity, which might be of prognostic value.