The authors sought to assess the utility of the electronic Medication Event Monitoring System (MEMS) in monitoring adherence to highly active antiretroviral therapy (HAART) in HIV-infected children and to compare this with other methods of adherence assessment. Twenty-six perinatally HIV-infected children being treated with three or more antiretroviral medications and their caregivers were enrolled and prospectively followed-up for 6 months. Adherence was assessed using MEMS monitoring of one antiretroviral, pharmacy refill records of all antiretrovirals, a caregiver self-report interview, a physician/nurse questionnaire, and appointment-keeping behavior. Viral loads measured at the end of the 6-month period were compared with the various adherence assessment methods. Adherence rates for the MEMS-monitored medication ranged from 12.7% to 97.9% (median = 81.4%), and 11 of the participants (42%) had less than 80% adherence using this method. A MEMS adherence rate greater than 80% was associated with viral load below the threshold of detection 6 months after enrollment (p <.001). Although not as robust, pharmacy refill rates for all antiretroviral medications were also associated with virologic response. The highest specificity was attained when both MEMS and pharmacy refill were used in combination. Physician assessment of adherence rate as well as appointment-keeping behavior was associated with virologic response, whereas caregiver self-report was not.