A clinical study is presented of 622 uric acid stone patients, constituting 34.6% of all patients with urolithiasis observed in a period of 12 years. The uric acid lithiasis in this series was essentially an upper urinary tract disease. Further evidence was obtained for idiopathic uric acid lithiasis being a disease entity, distinct from gouty uric acid lithiasis. Eighty percent of the uric acid stone patients were initially diagnosed as idiopathic uric acid lithiasis. Only 11 out of 359 patients with idiopathic uric acid lithiasis who came to follow up, developed gout. The majority of patients with idiopathic uric acid lithiasis were normouricemic and normouricosuric, only 13.6% of them became hyperuricemic during follow up. The male: female ratio in idiopathic uric acid lithiasis was 2.7:1, as compared with 12.3:1 in gouty uric acid lithiasis. In both the idiopathic and gouty uric acid lithiasis groups there was a preponderance of patients of European origin as compared with those of Near Eastern-Mediterranean origin. Myeloproliferative disease constituted a rare etiology in this series. There was a striking association of uric acid lithiasis with hyperparathyroidism, the latter occurring in 10.4% of the uric acid stone patients. Surgery and urinary tract infection was frequent.