Two hundred and twenty-one consecutive arthroscopic examinations of the knee are reviewed to establish diagnostic accuracy based on subsequent arthrotomy and the clinical course in the absence of arthrotomy. One hundred and nine knees showed an arthroscopic accuracy of 89.9 per cent at subsequent arthrotomy. Ninety-five patients who did not have an arthrotomy revealed 73.7 per cent clinical improvement between 9 months and 4 years after arthroscopy. Arthroscopy is a simple diagnostic procedure which, with experience, can be done safely and accuratly with minimal morbidity and, in this series, allowed 33.5 per cent of patients to avoid arthrotomy.