Nonadherence to antiretroviral therapy (ART) jeopardizes good clinical outcome in people living with HIV. In a single-center prospective study, prevalence and correlates of nonadherence were investigated in 43 patients on ART. Nonadherence was assessed using Medication Event Monitoring System (MEMS), self-report and collateral report of treating physicians. Based on MEMS data, median taking adherence, dosing adherence, and timing adherence was 98% (interquartile range [IQR] = 5.3), 91.5% (IQR = 18), and 86% (IQR = 31.5), respectively. The median number of drug holidays per 100 days was 0.8 (IQR = 4.8). The prevalence of nonadherence measured by MEMS was 40%. Self-reported nonadherence and collateral report of nonadherence by physicians varied from 5% to 41% and 24% to 28%, respectively. Patients were categorized as adherent or nonadherent based on a clinically validated algorithm derived from MEMS parameters. Nonadherent patients used significantly more escaping coping strategies (p = 0.003) and planned problem solving strategies (p = 0.049), were prescribed significantly more antiretroviral medications (p = 0.02) and were significantly longer on ART (p = 0.04) than adherent patients. Identified correlates of nonadherence may help clinicians in detecting patients with HIV at risk for nonadherence and can support the development of adherence enhancing interventions.