There are little data on antiretroviral therapy (ART) adherence among patients in Eastern Europe, despite the high incidence of HIV infection and the growing number of HIV-infected individuals who are being prescribed ART. The aim of this study was to measure rates of adherence to ART and factors associated with nonadherence among patients receiving care at an outpatient HIV clinic in Estonia. The study was based on cross-sectional data from a convenience sample of 144 patients receiving outpatient HIV care. Data were obtained via interviewer-administered surveys and data abstraction from clinical records. Adherence was measured from a 3-day patient self-report. Among 144 participants (mean age 33.8 years), two-thirds (63%) had been infected with HIV through intravenous drug use. Most (74%) were co-infected with hepatitis C (HCV). Perfect adherence over the last 3 days was commonly reported (88% [95% CI 81-92%]) with nonperfect adherence associated with greater concerns about the potential adverse consequences of taking ART (adjusted odds ratio [AOR] 4.8, 95% CI 1.2-34.0) and average (versus good/very good) self-reported health status (AOR 4.7, 95% CI 1.2-31.4). Self-reported ART adherence in this sample of Estonian HIV-positive patients in clinical care was similar to rates observed in Western Europe and other developed countries. Results suggest that adherence education and support may be most helpful if they specifically target the development of positive beliefs, reduction of negative expectancies towards ART.