['To investigate the effect of low-intensity pulsed ultrasound exposure on the healing of injured medial collateral ligaments.', 'Thirteen male Sprague Dawley rats were used in the study. After surgical transection of the bilateral medial collateral ligaments, the ligament of 1 knee received low-intensity pulsed ultrasound exposure (30 mW/cm2 for 20 minutes daily), whereas no ultrasound was applied to the contralateral knee (control side). Eight rats were killed at 12 days after surgery, and 5 rats were killed at 21 days. The bilateral knees of 5 rats were used for mechanical testing at each of the 2 periods, and 12-day specimens of the remaining 3 rats were prepared for the electron microscopic examination. The knees of 5 additional rats were used to obtain mechanical data of the normal uninjured medial collateral ligament.', 'On the 12th day, the low-intensity pulsed ultrasound-treated side exhibited significantly superior mechanical properties when compared with the control side in ultimate load, stiffness, and energy absorption (P < .05). However, the treatment did not afford any mechanical advantage when tested on the 21st day. The mean diameter of the fibril was significantly larger on the treatment side than on the control side (P < .05).'] Does low-intensity pulsed ultrasound enhance early healing of medial collateral ligament injuries in rats?