The reliability of clinical assessment of the knee was evaluated in 203 patients using magnetic resonance imaging (MRI). Neither the mechanism of injury nor clinical symptoms were useful indicators of the type of pathology. Physical signs proved insufficiently sensitive in detecting abnormalities. Overall, the accuracy of clinical diagnosis was 80.8 per cent for the anterior cruciate ligament, 62.9 per cent for the medial meniscus and 74.9 per cent for the lateral meniscus. Accurate investigations allow arthroscopy to be targeted to those likely to obtain therapeutic benefit. Reliance upon clinical judgement alone would have resulted in an 89 per cent increase in arthroscopic procedures. Investigations such as MRI or arthrography are concluded to be cost-effective methods of avoiding unnecessary hospitalization, morbidity and waste of limited resources.