Maintaining optimal adherence to antiretroviral therapy (ART) is essential for optimizing the management of HIV infection. The aim of this study is to explore ART adherence rates in Turkey. Included in this study were a total of 263 HIV-infected patients followed up by the ACTHIV-IST (ACTion against HIV in Istanbul) Study Group affiliated with four tertiary hospitals. The study population included patients 18 years of age or older who were on ART for over 12 months. Adherence was assessed by the medication possession ratio (MPR) calculated for each patient using data (a list of all drugs dispensed within the previous year for that patient) obtained from pharmacy medication records. In addition, patients completed a self-report questionnaire addressing missed doses and the AIDS Clinical Trials Group (ACTG) adherence questionnaire. The study was reviewed and approved by the Ethics Committee of Cerrahpasa Medical Faculty. Patient ages ranged from 19 to 71 years. Two hundred and thirty-one patients were male (88%). Two hundred and twenty-four patients (85%) had optimal adherence (MPR 95%). During the course of ART, 236 patients (90%) reported no missed doses in the past 4 days of their treatment, whereas 206 patients (78%) reported no missed doses in the past month. Simply forgetting was the most common reason for nonadherence. MPR was associated with virologic rebound. Major factors affecting adherence were being female, taking antituberculosis drugs, having an opportunistic infection, being able to take all or most of the medication as directed, and being aware of the need to take medication exactly as instructed to prevent the development of drug resistance. Adherence to ART measured by MPR and self-report surveys is relatively high in Turkey when compared with other countries, which probably led to high ART success rates.