We aimed to determine the initial adherence of HIV cohort patients to ART (antiretroviral therapy), and reasons for non-adherence. Patients who received ART at the time of enrollment in the Korea HIV/AIDS Cohort were included in this study. Treatment adherence was determined at the baseline interview by self-reported questionnaire. Eight-hundred thirty two HIV-infected patients received ART. Of these, 253 (30.4%) patients skipped ART more than once a month. The most common reason of skipping medication was “simply forgot†(60.4%).