Despite the absence of empirical evidence, serious mental illness is assumed to be a high risk factor for nonadherence to HIV antiretroviral regimens. To assess antiretroviral adherence among persons with serious mental illness, we conducted a study in which adherence was observed over a 2-week period with electronic monitoring bottle caps and self-report. Forty-seven participants enrolled, with all but two (96%) completing the study. Psychiatric diagnoses included bipolar depression (n = 24), schizophrenia (n = 12), schizoaffective disorder (n = 5), and major depression with psychotic features (n = 6). Mean adherence (proportion of prescribed doses taken) was 66% (standard deviation [SD] = 34), as measured by electronic monitoring; 40% demonstrated at least 90% adherence, but 31% had less than 50% adherence. Self-reported adherence to psychotropics was moderately correlated with self-reported (r = 0.45, p < 0.05) and electronically monitored (r = 0.39, p < 0.05) antiretroviral adherence. Viral load (log(10)) was negatively correlated with electronically monitored (r = -0.28, p < 0.10) and self-reported (r = -0.39, p < 0.05) antiretroviral adherence, after controlling for the length of time on treatment. These findings suggest that many patients with serious mental illness are able to adhere very well to antiretroviral regimens, yet a substantial proportion of our sample displayed poor adherence, indicating the need for research to further assess the factors that influence adherence to antiretrovirals in this population.