Low birthweight infants treated with chronic furosemide therapy are at risk for the development of intrarenal calcifications. A prospective longitudinal renal ultrasound investigation was conducted to study the correlation of diuretic therapy, clinical course and ultrasonographic findings. Of 117 premature infants studied ultrasonographically upon discharge from the hospital, 20 had intrarenal calcifications. Eight patients at age 16.3 +/- 2.6 months had sonographic resolution of renal calcifications, 6.6 +/- 1.1 months after furosemide therapy had been discontinued. Of the 12 patients with persistent calcifications, 4 died from severe pulmonary disease and autopsy in 3 of them confirmed the ultrasonographic diagnosis. All 12 children but 2 continued to receive furosemide for their chronic lung disease demonstrating significant association between chronic use of loop diuretics and persistence fo the renal calcifications (p less than 0.001). Two patients required nephrolithotomy and 4 suffered from recurrent urinary tract infections. In 4 patients, 5 kidneys were of small size and in 2 bilateral collecting system dilation was noted. We conclude that discontinuation of furosemide therapy is associated with resolution of the renal calcifications. On the other hand, continued treatment with furosemide is associated with high renal morbidity which indicates ongoing clinical and ultrasonographic follow-up.