We investigated the effects of local intraosseous application of basic fibroblast growth factor under physiologic conditions. An aqueous solution containing 0 (vehicle), 25, 100, or 400 micrograms of basic fibroblast growth factor was injected via a needle into the ilium of rats. Two weeks later, bone mineral density of the ilium was significantly increased (P < 0.01) with all three doses, and a dose-effect relationship was apparent. Light microscopy revealed proliferation of undifferentiated mesenchymal cells on the endosteal and trabecular surfaces, as well as apposition of newly formed bone on existing trabeculae. Intraosseous injection of basic fibroblast growth factor may be of use for the treatment of osteoporosis.