We have developed a radiological scoring scale for the evaluation of post-traumatic osteoarthritic changes after a knee ligament injury. It is based on radiographs taken from the knee at the time of injury, and from both knees at the follow-up examination. Ten criteria were used to devise and list the scale. The method has been used on 60 patients operated on primarily after an acute knee ligament injury. The mean age of the patients when injured was 36.6 years, and they were re-examined at an average of three and a half years after the operation. The scale includes adjustments so that the final scores measure only the pathological changes caused by the ligament injury. Reproducibility of the scale is excellent and the scores correlate well with the clinical results. The registration form is simple, may be completed easily and revised when necessary. The numerical data are readily prepared for statistical and computer analyses. Ninety five per cent of all the changes caused by the ligament injuries fitted the scale, showing success in the selection of the variables. The scale is recommended as a method of improving the uniformity and reliability of the assessment of post-traumatic osteoarthritic changes after knee ligament injury.