Swelling of the left knee in a 12 year old boy with a history of similar episodes involving both left and right knees. The differential diagnosis should include other possible causes of acute swelling of the knee. The finding of hemarthrosis in a known hemophiliac, confirmed by the discovery of bloodstained fluid on paracentesis of the left knee, along with the picture of muscle wasting above and below the right knee, make any other diagnosis unlikely. The immediate relief of symptoms on administration of antihemophiliac factor confirmed the diagnosis.