We applied arthroscopy in the diagnosis of thirty four patients with internal derangement of the knee from March to August, 1982. Its results were compared with clinical diagnosis and arthrographic findings. We also performed arthrofomy in twenty nine patients and the final diagnosis enabled us to evaluate the diagnostic accuracy of each method. The accuracy of clinical diagnosis was 63.4%. Arthrographic findings were correct in 77.8%. The diagnostic accuracy of arthroscopy was 92.3% with one false negative case. Unnecessary operations were avoided in five patients with the assistance of negative findings of arthroscopy. Arthrography and arthroscopy were complementary in the diagnosis of internal derangement of the knee.