Biopsy samples from five acquired immune deficiency syndrome (AIDS)-Kaposi's sarcomas and one non-AIDS-associated Kaposi's sarcoma were assayed by in situ RNA hybridization onto paraformaldehyde-fixed, paraffin-embedded skin sections for the presence of two fibroblast growth factor gene transcripts, FGFB and FGF5. FGF5 gene expression was detected in the characteristic Kaposi's sarcoma spindle-shaped cells in the five samples from human immunodeficiency-positive (HIV+) patients. FGFB transcripts were detected in Kaposi's sarcoma cells as well as in epidermis of HIV- and HIV+ patients. These results complement the observations about growth factor gene expression done on Kaposi's sarcoma-derived cell lines, which thus appear to be representative of what happens in vivo. Furthermore, they demonstrate a contrasting expression pattern of FGF5 and FGFB genes, both involved in the growth factor pathogenic cascade leading to Kaposi's sarcoma.