Previous studies have shown that age at onset of primary haemolytic uraemic syndrome (HUS) is a feature of prognostic significance, the disease being of much better outcome in paediatric patients younger than 3 years than in older children. In an attempt to find an explanation for such a difference, we analysed the clinical and pathological features of 42 children over 3 years of age who presented with HUS between 1955 and 1990 in our department. On the basis of the presence of a prodromal diarrhoea, we divided our patients into two groups: 21 children presented with the diarrhoea-associated (typical or D+) form of HUS, whereas 21 had the non-diarrhoea-associated (atypical or D-) form. Of the 42 children, 20 (47.5%) progressed to end-stage renal failure. However, our study shows that age at onset of HUS is not a prognostic feature per se. The difference in outcome between children and infants is most likely related to the high incidence of the atypical subset of HUS in children over 3 years, a subset that is very uncommon in infants. The ominous features which characterise this form of the disease are: (1) the absence of a diarrhoeal prodrome, (2) normal urine output, (3) marked proteinuria, (4) hypertension, (5) the occurrence of relapses or recurrences and (6) the presence of widespread and severe arteriolar changes on renal biopsy. The poor prognosis of the atypical form of HUS warrants the use of fresh-frozen plasma infusions and/or plasma exchange as early as possible in the course of the disease.