1. Arthrography of the knee is a valuable diagnostic procedure. In 225 consecutive knees examined and operated on by the same surgeon, the preoperative arthrograms, all performed and interpreted by the same radiologist, demonstrated meniscal tears correctly in 97.5 per cent of the 200 knees with such lesions, giving an accuracy of 99.7 per cent in the 143 lesions of the medial meniscus and of 93 per cent in the fifty-seven lesions of the lateral meniscus. Five tears found at operation were not demonstrated by arthrography, four in the lateral and one in the medial meniscus. In six patients, tears of both menisci, unsuspected on the basis of the clinical examination, were demonstrated correctly on the arthrograms. In no instance did an arthrogram indicate a tear in a meniscus which was not corroborated at surgery. In twenty-five patients with lesions other than meniscal tears, the arthrograms were helpful but not diagnostic for all types of lesions. 2. Prolonged observation and procrastination in the treatment of patients with symptomatic knees were avoided. The surgeon and patient were assured of a correct preoperative diagnosis, especially when the clinical signs were not specific and the symptoms were intermittent and not disabling. 3. Unnecessary surgery was reduced. 4. Arthrography revealed residual portions of the meniscus left after previous meniscectomy in eight patients, one of the retained fragments being itself torn. Reoperation resulted in cure in all eight patients. Osteochondral fractures were also demonstrated as well as loose fragments more than eight millimeters in diameter. 5. In this series, one-half of the thirty-four tears of the anterior cruciate ligament were demonstrated by arthrography. We are still developing the technique to demonstrate the cruciate ligaments on arthrograms. 6. Four patients with no anteroposterior instability were shown to have anterior cruciate tears by arthrography. 7. There were no complications of arthrography in this series. All patients returned to work immediately after the procedure.