Urinary saturation with respect to calcium oxalate monohydrate was measured in 111 consecutive patients with calcium nephrolithiasis. Each patient also was evaluated by a detailed conventional metabolic protocol. Patients with idiopathic hypercalciuria produced abnormally oversaturated urine more frequently than normal subjects and normocalciuric patients, but normocalciuric patients had unexpectedly high levels of urine saturation. Measuring levels of calcium concentration, oxalate concentration, or the chemical concentration product of calcium and oxalate in urine did not predict oversaturation. During thiazide treatment, saturation level tended to fall if it was initially elevated, whether the patient was hypercalciuric or not. Patients whose urine was not remarkably oversaturated showed no tendency to elaborate even less saturated urine during thiazide treatment; instead, the average calcium oxalate saturation level remained constant. Direct urine saturation measurements can detect a small but significant number of normocalciuric patients who have marked oversaturation with respect to calcium oxalate and appear to benefit from treatment.