Bone formation of fractured fibulae stimulated by ultrasound was evaluated with roentgenography and fluorochromes labelling. The fibulae of male New Zealand rabbits were fractured to create a 3 mm-gap. Ultrasounds at 0.5 or 1.0 W/cm2 were applied for various durations, 5, 15, and 25 min/day for 4 weeks. Ultrasonic treatments at 0.5 W/cm2, 1.5 MHz significantly accelerated bone formation at the fracture site for all durations, whereas the higher intensity at 1.0 W/cm2 suppressed the bone formation of the treated fractured fibula. Ultrasounds at 0.5 W/cm2 were stimulatory to fracture repair, if given for 15 min/day, and were recommended for future clinical trials. However, ultrasounds at the intensity of 1.0 W/cm2 were deleterious to the treated fracture and not recommended for clinical trials.