IMPORTANCE: Most children with end-stage kidney disease (ESKD) are treated with dialysis prior to transplant. It is not known whether their outcomes have changed in recent years. OBJECTIVE: To determine if all-cause, cardiovascular, and infection-related mortality rates for children and adolescents beginning dialysis improved between 1990 and 2010. DESIGN, SETTING, AND PARTICIPANTS: Retrospective cohort study of patients younger than 21 years initially treated with dialysis for ESKD, recorded in the United States Renal Data System between 1990 and 2010. Children with a prior kidney transplant were excluded. We used Cox proportional hazard models to estimate the hazard ratios (HRs) for mortality associated with a 5-year increment in year of ESKD treatment initiation. Primary analyses censored observation at kidney transplant. MAIN OUTCOMES AND MEASURES: All-cause, cardiovascular, and infection-related mortality. RESULTS: A total of 3450 children younger than 5 years and 19,951 children 5 years or older started dialysis from 1990-2010. Of those younger than 5 years, 705 died during dialysis treatment (98.8/1000 person-years); mortality rates were 112.2 and 83.4 per 1000 person-years in those initiating dialysis in 1990-1994 and 2005-2010, respectively. Of those 5 years and older at treatment initiation, 2270 died during dialysis treatment (38.6/1000 person-years). Their mortality rates were 44.6 and 25.9 per 1000 person-years in those initiating dialysis in 1990-1994 and 2005-2010, respectively. Each 5-year increment in calendar year of dialysis initiation was associated with an adjusted HR of 0.80 (95% CI, 0.75-0.85) among children younger than 5 years at initiation and an HR of 0.88 (95% CI, 0.85-0.92) among those 5 years and older. RESULTS: A total of 23,401 children and adolescents who initiated ESKD treatment with dialysis at younger than 21 years between 1990 and 2010 were identified. Crude mortality rates during dialysis treatment were higher among children younger than 5 years at the start of dialysis compared with those who were 5 years and older. Mortality rates for both children and adolescents being treated for ESKD with dialysis decreased significantly between 1990 and 2010. CONCLUSIONS AND RELEVANCE: In the United States, there was a substantial decrease in mortality rates over time among children and adolescents initiating ESKD treatment with dialysis between 1990 and 2010. Further research is needed to determine the specific factors responsible for this decrease.