Urinary tract infection (UTI) is a frequent cause of morbidity during the first years of life and may lead to renal insufficiency. Transforming growth factor-beta1 (TGF-beta) is both immunoregulatory and an important mediator of interstitial fibrosis. TGF-beta was detected in the urine of 52% of 48 children aged 1-24 months with a first episode of UTI (94% due to Escherichia coli) and no obstructive nephropathy compared with 0 of 20 healthy young children (P<0.001). TGF-beta was detected in the urine only during the early stage (<1 day) after initiation of treatment. It was detected more frequently (P=0.06) and in significantly higher concentrations (P=0.046) in children with a normal (99m )Tc-dimercaptosuccinic acid scan compared with those with abnormal scans performed 3-14 days after the diagnosis of UTI, suggesting a regulatory role in fibrogenesis and outcome of pyelonephritis in childhood.