Strict adherence to highly active antiretroviral therapy (HAART) is necessary for successful suppression of HIV replication. A large number of individuals are not adherent, however, and the reasons for non-adherence are varied and complex. We utilized cluster analyses to identify subgroups of adherers in a sample of 222 HIV positive individuals whose HAART use was electronically monitored. Five distinct subgroups were identified, with characteristic variations across the week and over the course of the 4-week study. Additional comparisons of demographic and behavioral variables found the worst adherers to have higher rates of substance use, and that a group with higher rates of cognitive impairment had a consistent drop in adherence during the weekends. In addition, the group with the best adherence had more individuals over the age of 50 years. The results of the current study indicate that distinct subgroups of adherers may exist, and suggest that interventions designed to improve adherence can be designed to accommodate this variability in behavior.