Basic fibroblast growth factor (bFGF) is a mitogenic and angiogenic polypeptide produced by diverse cell types, including the cells derived from normal tissue and neoplastic lesions. To evaluate the possible differential expression of bFGF in soft tissue tumors, we examined 17 frozen primary soft tissue tumors including extra-abdominal desmoids, leiomyosarcomas, synovial sarcomas, myxoid liposarcomas, and alveolar soft part sarcomas by nucleic acid in situ hybridization. Three of the four leiomyosarcomas and two of the three synovial sarcomas studied expressed bFGF mRNA, whereas all of the myxoid liposarcomas and alveolar soft part sarcomas were negative. Two of the leiomyosarcomas were also positive for FGF receptor mRNA. These results indicated the possibility of an autocrine or paracrine mechanism in the function of bFGF and also lent support to the opinion that in situ hybridization is useful for examining cell character.