The factors associated with a greater mortality risk in infants and young children undergoing dialysis have not been clearly determined. We report the results of a North American Pediatric Renal Transplant Cooperative Study designed to assess risk factors in patients aged younger than 6 years at initiation of dialysis therapy. Sixty-four nonsurvivors were matched with 110 survivors for age at dialysis initiation, primary renal disease, and year of entry onto the database. Questionnaires on 137 patients (51 nonsurvivors, 86 survivors) were completed by participating centers. Seventy-five percent (103 of 137 patients) of the patients were aged younger than 2 years at dialysis initiation; 42% (58 of 137 patients) had renal aplasia, dysplasia, and/or hypoplasia or obstructive uropathy; 62% were boys; and 62% were white. One-year patient survival rates were 83% in infants beginning dialysis at younger than 3 months of age, 89% in 3- to 23-month-olds, and 95% in 2- to 5-year-olds (P = 0.001). Comorbid nonrenal disease occurred in 37 of 51 nonsurvivors (74%) versus 46 of 84 survivors (55%; P = 0.027). Nonsurvivors had pulmonary disease and/or hypoplasia more often (14 of 37 nonsurvivors; 37.8% versus 8 of 46 survivors; 17.4%; P = 0.04). Oliguria or anuria was present in 23 of 33 nonsurvivors (70%) aged younger than 2 years versus 26 of 64 survivors (41%; P = 0.007). Infection accounted for 15 of 51 deaths (29.4%). In summary, these results suggest that age at dialysis initiation; presence of nonrenal disease, particularly pulmonary disease and/or hypoplasia; and oliguria or anuria in children aged younger than 2 years are identifiable as risk factors for mortality in these young patients.