The pseudogout syndrome has been suggested to be a diagnostic clue of hyperparathyroidism or a complication after parathyroidectomy that abruptly relieves hypercalcemia. In this report of 20 patients who had pseudogout, eight had intermittent attacks of arthritis associated with chondrocalcinosis that led through the pseudogout syndrome to a confirmed diagnosis of primary hyperparathyroidism. In an additional 12 patients, the diagnosis of pseudogout was made in patients who suffered from acute arthritis after parathyroidectomy that relieved the primary hyperparathyroidism. The postoperative attacks were seen most commonly on or after the second day after surgery and were associated with the lowest point in serum calcium levels. Pseudogout occurred in one or more joints, often involving the knee. The diagnosis was proved by aspiration of joint synovial fluid in many of the patients and identification of calcium pyrophosphate dihydrate crystals. To estimate the incidence of pseudogout in the population of patients with primary hyperparathyroidism, these 20 patients were distinguished in 531 patients undergoing parathyroidectomy, for a 3.8% incidence of the pseudogout feature. No other markers of higher risk for this complication were apparent, including age, sex, biochemistry, or prior history. This collected experience suggests that (1) pseudogout is a biochemical entity associated with primary hyperparathyroidism and may offer a useful clue to the recognition of the disease in screening; (2) acute arthritis after parathyroidectomy is most likely pseudogout, and this complication occurs more frequently than has been recognized previously; (3) patients with hypercalcemia have a high incidence of calcium pyrophosphate dihydrate crystal deposition in articular cartilage (chondrocalcinosis); and (4) relative hypocalcemia after parathyroidectomy is a stimulus for crystal shedding into synovial fluid, which may precipitate acute attacks of pseudogout.