Urinary citrate prevents nephrocalcinosis in adults. Premature infants have a high incidence of nephrocalcinosis. We measured urinary citrate (Ucit) and creatinine (Ucre) concentrations in infants with and without lung disease. Ucit was lower in the infants requiring mechanical ventilation than in the controls (mean = 9.7 vs. 15, p = 0.05). Ucit/Ucre ratios were lower in hypercarbic (mean = 0.95 vs. 1.47 in normocarbic, p less than 0.05) and acidotic (mean = 0.57 vs. 1.44 in nonacidotic, p less than 0.004) premature infants. The lowered Ucit of premature infants with lung disease (and acidosis or hypercarbia) may predispose to nephrocalcinosis by increased urinary lithogenesis.