The clinical and pathological characteristics of the acute stage of the HUS are described on the basis of 678 patients and the study of 52 autopsies and 3 kidney biopsies. The laboratory and pathological studies point to DIC as a basic pathogenic mechanism, probably related to a viral infection. The recent mortality rate has been 6.25% due to the exacting dialysis treatment employed. Heparin was ineffective in the 26 patients in which it was used. 124 patients have a follow-up of more than 5 years and up to 13 years. 60 patients are completely recovered, while 64 have persistent signs of renal disease. Nine other children died in uremia 1–12 years after onset. The pathological findings are discussed on the basis of 6 autopsies and 24 biopsies in 27 patients. This stage of the HUS appears as an important and new aspect of pediatric nephrology.