Thirty-nine patients underwent reconstruction of the anterior cruciate ligament with carbon-fibre and a MacIntosh repair; all had a negative pivot shift test after operation. Some patients had persistent pain, mild effusion and synovial thickening; in 10 of these patients the symptoms warranted arthroscopic examination and biopsy at a mean of 16.9 months after the repair. Arthroscopy revealed that the carbon-fibre had not induced the formation of a "new ligament" and that the repair was merely covered by a thin, fibrous sheath. Histological investigations confirmed this finding, with only a suggestion of a fibroblastic response to carbon-fibre found in two patients. Particles of carbon-fibre were found scattered through the knees. Synovitis and breakdown of the skin over subcutaneous carbon-fibre complicated treatment. Failure of the carbon-fibre to bond to bone was detected radiographically.