Untreated anterior cruciate ligament injuries in 97 knees of 87 patients were reviewed after a mean interval of six years. After their initial recovery 63% of the patients were able to return to their pre-injury sport, but six years later 27% of them had deteriorated to the extent that they could not cope with the same level of sport. Radiological deterioration was maximal in those that had had a meniscectomy more than five years before review. Knees with intact menisci were often radiologically normal despite continuing instability.