Insulin-like growth factors (IGFs) are potent mitogens for a variety of cancer cells in vitro. A paracrine/autocrine role of IGF-II in the growth of breast and prostate cancer cells has been suggested. Information on cell-type-specific IGF-II expression in vivo in the breast and prostate is, however, limited. Thus, cell types expressing IGF-II mRNA and protein in tumors were identified by in situ hybridization and immunohistochemistry. Of 36 prostate, 17 breast, and 10 bladder cancers, and 9 paraganglioma tissues examined, IGF-II was expressed in more than 50% of prostate, breast, and bladder tumors, and in 100% of paraganglioma tumors. Expression levels of IGF-II were highest in the paraganglioma and bladder followed by prostate and breast tumors. In all the tumors expressing IGF-II, both mRNA and protein were localized to malignant cells, expression in the stroma being minimal. Since previous studies had indicated that an incompletely processed form of 15-kDa IGF-II exhibited higher mitogenic potency than the completely processed 7.5-kDa IGF-II form, the quantity and size of IGF-II proteins expressed in these tumors were analyzed by Western immunoblotting. Greater expression of 15-kDa IGF-II relative to the 7.5-kDa IGF-II form was clearly demonstrated in all six prostate cancers and in half of the two breast and four bladder cancers examined. The results are consistent with the hypothesis that the 15-kDa form of IGF-II expressed in cancerous cells contributes to autocrine cancer cell growth in vivo.