The authors performed 396 knee arthroscopies on athletic patients under general or spinal anesthesia. One hundred-fifty patients underwent arthroscopy after a careful history and physical examination. The clinical diagnosis was correct in 97 patients, correct but incomplete in 25 patients, and incorrect in 28 patients. Our total clinical accuracy rate was 71%. Arthroscopy increased accuracy from 71 to 97%. The remaining 153 patients had knee arthrography before undergoing diagnostic arthroscopy. The arthrographic accuracy rate for the medial compartment was 63% and for the lateral compartment was 36%. Arthroscopic accuracy rate for these patients was 95% for the medial and 97% for the lateral compartments. Two hundred of the 250 arthroscopies were followed by immediate arthrotomy. There were no deep or superficial infections. Arthroscopy, when compared to our clinical impression and arthrography, was the most accurate diagnostic aid. Using it we made the correct diagnosis in 392 out of 396 patients.