ObjectiveTo determine whether adherence with one antiretroviral medication reliably predicts adherence with other antiretroviral medications.DesignCross-sectional repeated-measurement analysis.SettingCohort study enrolling patients from Massachusetts and Rhode Island.PatientsTotal of 454 patients using antiretroviral medications for HIV infection.MethodsFor each antiretroviral medication, we asked patients to report how many days in the last week they had 1) missed a dose of the medication and 2) been off schedule with a dose of the medication. The reliability coefficient was used to compare between-person variance in adherence rates with total variance, which is the sum of between-person and within-person variance.ResultsThe mean age of patients was 42 years; 28% were women and 37% were nonwhite. Sixty-six percent of patients were on three or more antiretroviral medications. Perfect adherence was reported by 42% of patients, and patients reported missing a dose on a mean of 1.5 days per week for each antiretroviral medication currently being taken. The reliability coefficient for days in the last week that the medication was missed was 0.85 (95% confidence interval [CI]: 0.83–0.87), and for days off-schedule in the last week, it was 0.88 (CI: 0.86–0.89).ConclusionsMost of the variability in antiretroviral adherence in this study was accounted for by between-patient differences in overall adherence rather than by within-patient differences in adherence patterns across medications. These data support the theory that when patients skip or are off schedule with doses, they skip or are off schedule with all the antiretroviral medications taken at that time. In the course of exploring patients' adherence issues, clinicians may find it useful to inquire about problematic dosing times. Researchers assessing adherence may not need to separately monitor adherence with each antiretroviral medication in a regimen.