We hypothesized that extracorporeal shock waves induce overgrowth and local increases in bone mineral content (BMC) in immature long bones. Immature male rabbits (n=14; 9 weeks old) were randomized equally between group I, which received 1000 100 MPa shock waves on the femoral shaft and group II, which received 5000. Unexposed femurs were used as controls. No fractures occurred in group I; three occurred in group II. Six weeks after exposure, the length and width were significantly larger (1.0 and 14.9%, respectively), and the BMC was significantly higher (22.8%) than those of control femurs in group I. These results of differences in width and BMC might be clinically useful.