Between January 1990 and December 1991, serial real-time ultrasound examinations and analyses of urine were performed on a total of 50 infants with birth weights less than 1,500 g to assess the incidence of renal calcification. Five infants (10%) developed renal calcification at a mean age of 48.8 ± 14.1 days. These 5 infants with renal calcification had significantly shorter gestations (28.2 ± 0.8 vs. 30.1 ± 1.7 weeks, p < 0.0005) and lower birth weights (934 ± 45 vs. 1,311 ± 188 g, p < 0.0005) when compared with infants without renal calcification. None of the affected infants were treated with furosemide. Affected infants had a mean urine volume of 85.8 ± 11.3 ml/kg/24 h, mean urine calcium level of 5.07 ± 1.18 mg/kg/24 h, mean urine calcium to creatinine (mg/mg) ratio of 0.67 ± 0.09, and a mean urine N-acetyl-β-D-glu-cosaminidase (NAG) to creatinine (U/g) ratio of 259 ± 133. Urinalyses showed that affected infants had significantly higher urine pH values and hematuria. Alkaline phosphatase concentrations and initial parathyroid hormone levels were not different among the two groups. In summary, renal calcification occurred in 10% of very low birth weight infants and multiple risk factors seem to be contributory. The smaller, sicker and more immature infants appear to have increased risk for developing renal calcification. For earlier detection and treatment of renal calcification, follow-up screening by serial ultrasound examinations and assay of urinary excretion of calcium, creatinine and NAG are useful, and a meticulous search for the causes of renal calcification is mandatory.