From 1970 through 1971, 31 episodes of pseudogout occurred in 28 hospitalized patients. Twenty-three episodes occurred after surgery (21 operations, including 11 orthopedic procedures and 3 parathyroid adenectomies) at a median of three days. Five of the nine attacks in medical patients occurred after acute vascular illnesses (median, seven days). Knee involvement was most common (25 of 31 episodes); other large joints were sometimes affected, and nearly half of the patients had more than one joint inflamed. Ten of the patients were observed during their first episode. Joint aspiration with intra-articular steroid injection appears to be the best treatment. Because most attacks were noted during the early postoperative phase, hypocalcemia is postulated as one metabolic "trigger" for this form of crystal synovitis.