Basic fibroblast growth factor (bFGF) is an angiogenic and mitogenic polypeptide produced by diverse cell types including cell lines derived from malignant melanomas but not from normal melanocytes. However, there is no consensus concerning in vivo expression of bFGF in melanocytic lesions due in part to the small numbers of cases studied to date. To evaluate further the possible differential expression of bFGF in melanocytic lesions, we examined 110 formalin-fixed, paraffin-embedded metastatic and primary invasive melanomas, melanomas in situ, nevi with architectural disorder and cytological atypia, and ordinary benign melanocyte nevi by nucleic acid in situ hybridization. All metastatic and primary invasive melanomas studied expressed bFGF mRNA, whereas melanomas in situ and benign melanocyte nevi were negative. Melanomas in situ with features of tumor regression and a majority of nevi with architectural disorder and cytological atypia also contained bFGF mrNA. The results suggest that in vivo bFGF expression is not requisite for malignant transformation per se, but appears to correlate more with invasion or fibroblastic reactions adjacent to the melanocyte lesions.