To assess mortality and loss to follow-up of children with HIV infection who started antiretroviral therapy (ART) through the Universal Coverage Health Program (UC) in Thailand.Children with HIV infection who initiated ART at age <15 years through the UC between 2008 and 2013 were included in the analysis. Death was ascertained through linkage with the National Death Registry. A competing-risks method was used to calculate subdistribution hazard ratios (SHRs) of predictors for loss to follow-up. Death was considered a competing risk. Cox proportional hazards models were used to assess predictors of mortality.A total of 4618 children from 497 hospitals in Thailand were included in the study. Median age at ART initiation was 9 years (IQR, 6-12 years), and the median duration of tracking was 4.1 years (a total of 18 817 person-years). Three hundred and ninety-five children (9%) died, for a mortality rate of 2.1 (95% CI, 1.9-2.3) per 100 person-years, and 525 children (11%) were lost to follow-up, for a lost to follow-up rate of 2.9 (95% CI, 2.7-3.2) per 100 person-years. The cumulative incidence of loss to follow-up increased from 4% at 1 year to 8.8% at 3 years. Children who started ART at age ≥12 years were at the greatest risk of loss to follow-up. The probability of death was 3.2% at 6 months and 6.4% at 3 years. Age ≥12 years at ART initiation, lower baseline CD4%, advanced HIV staging, and loss to follow-up were associated with mortality.The Thai national HIV treatment program has been very effective in treating children with HIV infection, with low mortality and modest rates of loss to follow-up.