We describe the clinical and laboratory features of 20 children who were seen during the past 20 years with idiopathic nondiarrhea-associated hemolytic-uremic syndrome. There was no seasonal variation in time of onset; a genetic pre-disposition seemed likely in two of the cases. The prodromal illness was nonspecific and by definition did not include diarrhea. Hypertension was a major problem in the majority of the patients. Five died, three during the initial illness; four are in end-stage renal failure, and all but two of the survivors have residual nephropathy. Eleven patients had a "relapsing" course; up to eight additional documented episodes of hemolytic-uremic syndrome occurred in individual patients. Of the nine children treated before 1980, three died shortly after onset, two never recovered function after the initial illness, one had a relapsing course and died later, and one had residual nephropathy. Plasma exchange was introduced for the management of non-diarrhea-associated hemolytic-uremic syndrome in 1980; since then, all of the 11 patients have recovered function after the initial episode, but 10 of them had relapses. It appears that with the introduction of plasma exchange there has been an improved outcome in the initial phase, but the survivors tend to have relapses. Atypical (non-diarrhea-associated) hemolytic-uremic syndrome is a heterogeneous yet distinct subgroup of hemolytic-uremic syndrome that differs from diarrhea-associated hemolytic-uremic syndrome on epidemiologic, clinical, laboratory, histologic, and prognostic grounds.