AbstractHemolytic uremic syndrome (HUS) is characterized by hemolytic anemia, thrombocytopenia and acute renal insufficiency. Recently we experienced 6 HUS patients who had different clinical modes of onset, different clinical courses and a poor prognosis compared with the patients with the above typical clinical pictures. Thirteen HUS patients were evaluated in two aspects: mode of onset (acute or gradual type) and clinical corn (non‐recurrent or recurrent type).During 4.9 ± 3.1 (mean ± sd) years' of observation, renal insufficiency was more frequently observed in the gradual onset and recurrent course types respectively than in the acute onset and recurrent types (p < 0.05, p < 0.01). Also, the mean age was significantly lower and platelet counts were much higher in the gradual onset type than in the acute onset type (p < 0.05, p < 0.01). However, clinical findings except renal failure revealed no differences between non‐recurrent and recurrent HUS. For the prediction of HUS, we emphasize the importance of classifying HUS by the mode of onset (acute or gradual) and/or clinical course (non‐recurrent or recurrent).