Thailand began a national antiretroviral (ARV) treatment program in 2000, and all government and some private and university hospitals now provide treatment to eligible HIV-infected patients. We describe program scale-up and patient outcomes from 2000 to 2007. Data from 839 hospitals in all 76 provinces of Thailand were included in this analysis. Outcomes were assessed for patients initiating ARV treatment from January 2000 to December 2005. Follow-up data through March 2007 were included; lost to follow-up was defined as >3 months late for a follow-up visit. A Cox proportional hazard model was used to assess risk factors for death; the Kaplan-Meier method was used to estimate survival probabilities. Outcome data are reported for 58,008 patients. Among these, 52.2% were male; at treatment initiation, the median age was 34 years, the median CD4 count was 41 cells per cubic millimeter, and 50.5% had AIDS. The initial regimen was nevirapine and 2 nonnucleoside reverse transcriptase inhibitors for 92.4% of patients; median follow-up time was 1.6 years (interquartile range = 0.8-2.4 years). Lost to follow-up occurred in 8.8% of patients. Overall 1-year survival was 0.89 (95% confidence interval = 0.88 to 0.89). Death was significantly associated with male sex, age >40 years, baseline CD4 count <100 cells per cubic millimeter, symptomatic HIV or AIDS, receipt of services at a district or community hospital, and treatment initiation before 2005. National ARV treatment programs can be scaled up rapidly with good patient outcomes. Treatment outcomes among patients in Thailand are comparable to those reported in smaller cohorts in other countries, and survival rates have improved since 2004.