By direct measurement, urine from patients who form calcium oxalate stones was supersaturated abnormally with respect to calcium oxalate monohydrate but not supersaturated with respect to undissociated uric acid. Urine from patients who form uric acid stones was supersaturated excessively with undissociated uric acid but not calcium oxalate. Patients who form both calcium oxalate and uric acid stones, however, produce urine that is supersaturated with respect to both solid phases. Low urine pH was the primary factor that increased supersaturation with respect to undissociated uric acid. The formation of both calcium oxalate and uric acid stones appears to be explained by a dual abnormality of urine supersaturation.