The healing of anterior cruciate ligaments reconstructed with the Leeds-Keio artificial ligament was observed by arthroscopy in 42 knees and biopsy in 19 knees at intervals from 3 to 24 months after implantation. By three months the implant was covered with immature new tissue, and a dense vascular network crossed its surface. At 12 months a new ligament had developed and matured, looking like the natural one in most cases. Histology at this stage showed abundant collagenous fibres running parallel and longitudinally, while the synovial membrane showed no more than very slight inflammatory changes. By 18 to 24 months, the new ligament often had the arthroscopic appearance of a normal anterior cruciate ligament. These results suggest that this scaffold type of artificial ligament is effective for cruciate reconstruction, giving satisfactory healing without significant complications.