Acidic fibroblast growth factor (aFGF) is a regulatory peptide which, on account of its structural homologies with the products of oncogenes, is involved in cell proliferation, differentiation, and motility. We previously reported the presence of aFGF in the urine of patients with transitional cell carcinoma (TCC). aFGF can also induce the motility of a rat-derived bladder carcinoma cell line (NBTII). This immunohistochemical study used polyclonal rabbit antibodies against acidic and basic FGF and peroxidase detection. Native NBTII nude mice xenografts and aFGF transfected NBTII (NFS14) nude mice xenografts were used as tissue controls for antibody specificity. The samples included 4 normal urothelia and 12 TCC. In addition, cytospins of 4 different tumoral cell lines of human bladder and normal bladder cells were stained. The results showed strong immunostaining in all tumoral urothelium samples using anti-aFGF and a very low amount of staining or none at all in healthy tissues. A primary analysis suggested that the strongest reaction was obtained in high-grade tumors (3 + vs + for lower-grade tumors). Using bFGF antibody, strong immunohistochemical staining was detected on basal membranes and stromal vessels and none in urothelium. These data confirm aFGF expression in the epithelial cell compartment of bladder cancer and the likely involvement of this regulatory peptide in the biology of TCC.