CD95 ligand (CD95L) potently induces apoptosis by activating CD95 on target cells. It has recently been reported that melanoma cells in vivo express a significant amount of CD95L, thereby being immediately able to kill CD95-bearing immunocompetent cells specific for cancer antigens, which infiltrate the lesions. In this study, we employed immunohistochemistry using an antibody directed against CD95L to investigate at which stage the melanoma CD95L expression is turned on. Skin biopsies of 49 lesions from 46 patients were assessed. These included benign and dysplastic naevi, melanoma in situ, stage I melanomas (Clark's level 2 or 3), advance-phase melanomas (Clark's level 4 or 5) and lymph node metastases. CD95L was expressed in all of the advance-phase melanomas as well as lymph node metastases of cutaneous origin, whereas neither melanoma in situ, benign naevi nor dysplastic naevi reacted positively with the antibody. To investigate a link between positivity and tumour size, the data were analysed on the basis of Breslow thickness, and indicated that expression was observed only when tumours were thicker than 0.75 mm. We next compared expression of CD95L and HMB-45. CD95L was positive only in melanomas in a more advanced phase than stage I, whereas HMB-45 was not only expressed in melanoma cells but also in benign pigmented naevi. This indicated the advantage of CD95L staining to diagnose melanoma. The present study indicates the significant correlation between tumorigenicity and expression of CD95L, and thereby raises the possibility that CD95L may be a useful diagnostic marker for malignant melanomas.