New treatments for HIV can improve immune functioning and decrease mortality. However, lapses in adherence may render these complex regimens ineffective. Sixty-three men and 9 women on highly active antiretroviral therapy completed measures of medication adherence, psychological characteristics, and barriers to adherence. HIV viral load, a health outcome measure of virus amount present in blood, was also obtained. The sample was 36% African American and 56% Caucasian, with 35% reporting disability. Nearly one third of patients had missed medication doses in the past 5 days, and 18% had missed doses weekly over the past 3 months. Frequency of missed doses was strongly related to detectable HIV viral loads. Depression, side-effect severity, self-efficacy, and social support distinguished patients with good and poor adherence. Barriers also varied with adherence level. Implications for interventions promoting HIV treatment adherence are discussed.