Sex is a well proven prognosticator in primary malignant melanoma. We studied 19 parameters of tumor cells in primary malignant melanoma from 391 patients, the purpose being to determine if melanomas in men and women are alike. Apart from results in thin melanomas, no difference was seen between the sex groups. Thin melanomas showed a significant difference for the melanoma-associated antigen G7-E2 (higher expression in women) and the histocompatibility antigen HLA-DR (higher expression in men). The results suggest that sex-related prognostic differences are due more to host than to tumor characteristics.