Plasma renin activity (PRA) was measured in 50 consecutive patients (aged 4 months to 12 years) admitted during the acute phase of the haemolytic uraemic syndrome (HUS). Blood samples were taken as soon as the diagnosis was made and prior to any diuretic, anti-hypertensive or dialysis treatment. Prodromal diarrhoea was present in all but 3 patients, 17 were anuric and 12 were oliguric. PRA ranged from 0.3 to 24.2 ng/ml per hour and was low compared with values in normal infants and children: in 13 HUS patients PRA was above the median and in 37 it was below the median. PRA was significantly, independently and inversely related to age. There was no correlation, however, with blood pressure, urine output, volume status and serum levels of sodium, potassium and creatinine. Moreover, no relationship was found between PRA and the course of the disease. Our findings do not support the idea that renin activation plays a role in the pathophysiology of the haemolytic uraemic syndrome.