The CD10 antigen is a neutral endopeptidase expressed by a variety of mesenchymal tumours (haemopoietic or not), including a subset of malignant melanomas. We investigated the expression of CD10 in formalin-fixed, paraffin-embedded tissue specimens of 72 cutaneous melanomas (28 primary, 26 metastatic to the skin and 18 lymph node metastases). The CD10 antigen was expressed by 18 of the 26 (69%) and 11 of the 18 (61%) melanomas metastatic to the skin or lymph nodes, respectively; in contrast, only six of the 28 primary melanomas (21.4%) expressed appreciable CD10 reactivity, and expression was usually lower (in terms of the percentage of immunoreactive cells) than that found in metastatic tumours. The sensitivity, specificity, and positive and negative predictive values of CD10 positivity for metastatic malignant melanoma were calculated to be 0.66, 0.79, 0.83 and 0.6, respectively. Our results suggest that the CD10 antigen is upregulated during the process of metastasis in melanomas. From a diagnostic point of view, the expression of CD10 appears to be an additional feature for the histological differential diagnosis between primary and metastatic melanomas.