BACKGROUND: In recent years, Bulgaria has increasingly relied on out-of-pocket payments as one of the main sources of health care financing. However, it is largely unknown whether the official patient charges, combined with informal payments, are affordable for the population. Our study aimed to explore the scale of out-of-pocket payments for health care services and their affordability. METHODS: Data were collected in two nationally representative surveys, conducted in Bulgaria in 2010 and 2011, using face-to-face interviews based on a standardized questionnaire. To select respondents, a multi-stage random probability method was used. The questionnaire included questions on the out-of-pocket payments for health care services used by the respondent during the preceding 12 months. RESULTS: In total, 75.7% (2010) and 84.0% (2011) of outpatient service users reported to have paid out-of-pocket, with 12.6% (2010) and 9.7% (2011) of users reporting informal payments. Of those who had used inpatient services, 66.5% (2010) and 63.1% (2011) reported to have made out-of-pocket payments, with 31.8% (2010) and 18.3% (2011) reporting to have paid informally. We found large inability to pay indicated by the need to borrow money and/or forego services. Regression analysis showed that the inability to pay is especially pronounced among those with poor health status and chronic diseases and those on low household incomes. CONCLUSION: The high level of both formal and informal out-of-pocket payments for health care services in Bulgaria poses a considerable burden for households and undermines access to health services for poorer parts of the population.