Basic fibroblast growth factor (bFGF or FGF-2) is abnormally elevated in the serum and urine of patients with many types of cancer. However, the source of the bFGF is unclear. We developed a model that could distinguish between tumor-derived and host-derived bFGF. We gave athymic mice s.c. injections of cells of the murine K1000 tumor, which secretes a bFGF mutein (bFGF CS23) as its dominant angiogenic factor. Controls were given injections of Lewis lung carcinoma or saline. Urine was collected for 9 weeks, and bFGF was quantitated using two immunoassays which can discriminate between tumor bFGF CS23 and native bFGF. None of the mice had detectable urinary native bFGF, and no control mice had detectable urinary bFGF CS23. In contrast, urine from mice bearing the K1000 tumor revealed detectable bFGF CS23 by 2 weeks, and bFGF CS23 increased with increasing tumor volume throughout the study. Because bFGF CS23 is not produced by other cells, the bFGF CS23 in the urine most likely came from the K1000 tumor and not from the host. These results suggest that the source of elevated bFGF in the urine of human cancer patients is, at least in part, the tumor itself.