Several investigators have reported that hypocitraturia is frequent in patients with idiopathic kidney stones. In these studies, however, glomerular filtration rate, urinary tract infection, sex, diet, time of day, and medications, all potentially influential variables, were uncontrolled. Fifteen men, aged 30-52 yr, with recurrent idiopathic calcium oxalate stones and 15 normal age-matched men were studied. Patients with hyperparathyroidism, renal tubular acidosis, reduced creatinine clearance (less than 80 ml . min/1.73 M2), or urinary infection were excluded. Medications were stopped 2 weeks before the study began. A standard constant diet, furnishing 800 mg calcium and free of citrate, was fed for 20 days. During the last 10 days, 4.5 g sodium citrate were given orally. Eight-hour collections of urine were analyzed for calcium and citrate. Filtered load and net tubular reabsorption of citrate were also calculated. The 24-h urinary excretion of calcium was elevated in eight stone formers, and citrate excretion was depressed in seven. Five patients were both hypercalciuric anc hypocitraturic. The hypocitraturia resulted from excessive net tubular reabsorption of a normal filtered load of citrate. Urinary citrate was highest between 0800-1600 h, whereas calcium was highest between 1600-2400 h; both components were lowest between 2400-0800 h. The diurnal profiles of urinary calcium and citrate were similar in the stone formers and in the normal men. Oral sodium citrate did not influence urinary citrate in either group. These data suggest that in adult men, hypocitraturia may be a common predisposing factor for calcific nephrolithiasis.