The patellar tendon donor site of 20 patients who underwent anterior cruciate ligament (ACL) reconstruction using the patellar tendon tissue as autograft was examined with high resolution 7.5 MHz ultrasound. The patients were randomly divided into four groups and studied at 3, 6, 9 or 12 months postoperatively. The size of the postoperative tendon defect was measured just distal to the lower pole of the patella. The size of the tendon defect diminished progressively from a mean of 109 mm2 at 3 months to a mean of 23 mm2 at 12 months. Increasing echogenicity was first noticed 12 months after tendon repair. Seven patients developed clinical features postoperatively of jumper's knee (patellar tendinosis). There were no ultrasound signs that differentiated these patients from asymptomatic patients. It is concluded that ultrasound provides objective evidence of patellar tendon healing after ACL reconstruction: the surgical defect diminished in size and became echogenic after a period of 12 months.