OBJECTIVE: To assess out-of-pocket payments and catastrophic health expenditures among antiretroviral therapy (ART) patients in Vietnam, and to model catastrophic payments under different copayment scenarios when the primary financing of ART changes to social health insurance. METHODS: Cross-sectional facility-based survey of 843 patients at 42 health facilities representative of 87% of ART patients in 2015. RESULTS: Because of donor and government funding, no payments were made for antiretroviral drugs. Other health expenditures were about $66 per person per year (95% CI: $30-$102), of which $15 ($7-$22) were directly for HIV-related health services, largely laboratory tests. These payments resulted in a 4.9% (95% CI: 3.1-6.8%) catastrophic payment rate and 2.5% (95% CI: 0.9-4.1%) catastrophic payment rate for HIV-related health services. About 32% of respondents reported, they were eligible for SHI without copayments. If patients had to pay 20% of costs of ART under social health insurance, the catastrophic payment rate would increase to 8% (95% CI: 5.5-10.0%), and if patients without health insurance had to pay the full costs of ART, the catastrophic payment rate among all patients would be 24% (95% CI: 21.1-27.4%). CONCLUSIONS: Health and catastrophic expenditures were substantially lower than in previous studies, although different methods may explain some of the discrepancy. The 20% copayments required by social health insurance would present a financial burden to an additional 0.6% to 5.1% of ART patients. Ensuring access to health insurance for all ART patients will prevent an even higher level of financial hardship.