We examined the clinical course of 17 preterm infants with chronic lung disease who received loop diuretics and developed nephrocalcinosis; nine of them were followed for up to 4.5 years. The mean gestational age was 26.8 weeks (SD 2.2 weeks), and mean birth weight was 830 gm (SD 276 gm). The diagnosis of renal calcification was made at a mean age of 12 weeks (SD 6.5 weeks) by abdominal x-ray examination, screening abdominal ultrasound studies or, both. Calcification was associated with both furosemide therapy and the presence of multiple potential risk factors. Renal calcification, length, and function were subsequently evaluated in nine patients at a mean age of 21.3 months (SD 15.3 months). Improvement in calcification occurred in five patients, with total resolution in four. Renal length, determined by ultrasound examination and corrected for body length, was normal in 17 of 18 kidneys. Serum creatinine values and calculated glomerular filtration rates were abnormal in four of nine patients. We conclude that renal calcification in preterm infants is associated with multiple risk factors, including furosemide usage, and tends to diminish during the first year of life. However, renal function may remain compromised in some patients.