The risk of nephrocalcinosis in preterm infants is considerable, but conflicting numbers are given for the actual incidence (10-65%). Furosemide induced hypercalciuria is said to be the main risk factor. We examined retrospectively the incidence, causes and outcome of nephrocalcinosis in preterm infants born in our hospital from 1988 to 1998 ( n=2190). An abnormal renal echogenicity or nephrocalcinosis was seen in 31 infants (29.7+/-3.3 weeks gestational age; 1307+/-690 g birth weight). Nephrocalcinosis was diagnosed in 16, hyperechoic kidneys (HK) in 10 and Tamm-Horsfall kidneys in 5 infants. Main risk factors were low gestation age and birth weight, length of hospitalization, variations in acid-base status, length of assistant ventilation and hypercalciuria at diagnosis. The incidence of nephrocalcinosis was 0.73% [1.7% for low birth weight infants (VLBW)]. Taking the cases of nephrocalcinosis and HK together, incidence was calculated to be 1.2% overall and 2.5% for VLBW infants, but increased to 7% in 1998. The follow-up showed persisting nephrocalcinosis or hyperechoic kidneys in 8/26 preterm infants. In conclusion, the incidence of nephrocalcinosis was lower in our population than is usually reported. The numbers have, however, increased over the past few years. From the follow-up it was obvious that long-term observation of preterm infants is necessary and that complications might arise in the long run.