Out of 386 unicompartmental prostheses inserted in the Orthopaedic Centre in Dracy Le Fort between 1977 and 1986, 79 cases in 68 patients with a follow-up equal to or more than five years and a mean of seven years were able to be reviewed. This study showed that 75 per cent of knees had a satisfactory result and 25 per cent were failures. Apart from well-recognised causes of failure such as rheumatoid arthritis, overcorrection or the use of a six millimetre tibial plateau, the authors discovered a frequent cause not clearly recorded in the literature--that is anterior cruciate laxity. This situation was found retrospectively in the pre-operative weight-bearing radiographs. Out of 15 knees showing a pre-operative laxity equal to or greater than 10 mm in the lateral radiograph, 13 were failures and 10 were operated on again after a mean of three-and-a-half years. An investigation of anterior cruciate laxity should be made systematically by weight-bearing lateral radiographs, if necessary with stress views, before recommending a unicompartmental prosthesis.