The urinary citrate excretion was examined in patients with nephrolithiasis who were categorized on the basis of different physiologic or metabolic abnormalities. A wide prevalence of low citrate excretion (hypocitraturia) was observed, with over one half of our patients with stones exhibiting it. Hypocitraturia was found in all patient categories except primary hyperparathyroidism and hyperuricosuric calcium oxalate nephrolithiasis. As expected, hypocitraturia was present in renal tubular acidosis and in enteric hyperoxaluria. However, urinary citrate was also low in absorptive and renal hypercalciurias, and in patients in whom an acid-base disturbance was clearly excluded.