Three boys aged 4, 5 and 7 weeks drank poorly, vomited and were lethargic. There were metabolic disorders attributable to a urinary tract infection. Ultrasonography revealed anatomical anomalies. After antibiotic treatment and, if necessary, surgical correction, the patients recovered. Follow-up was uncomplicated except persisting polyuria in one of the patients. A urinary tract infection in young children is difficult to recognise because of the aspecific presenting symptoms. It can cause a severe metabolic disturbance in which hyponatraemia and hyperkalaemia develop (pseudohypoaldosteronism), combined with metabolic acidosis and polyuria. A high alertness for urinary tract infections in young children with these aspecific symptoms is needed as well as metabolic and urologic evaluation.