Pharmacologic boosting of protease inhibitors with ritonavir has become standard practice in antiretroviral therapy. Patients are instructed to take ritonavir at the same time as its accompanying protease inhibitor. However, ritonavir is unpopular with many patients because of its large size and because of the recommended need for refrigeration. This study will test the hypothesis that adherence to ritonavir is inferior to adherence to its accompanying protease inhibitor in patients receiving such therapy.

The study staff will employ MEMS caps to prospectively measure adherence to ritonavir and its accompanying protease inhibitor over 24 weeks of follow-up. Paired sample t-tests will be employed to compare adherence to the two agents, and secondary analyses will be conducted to evaluate the chronologic concordance of ritonavir and accompanying protease inhibitor dosing.

Inclusion Criteria:

• HIV infection, age>18 years, receiving therapy with ritonavir-boosted atazanavir or fosamprenavir, willingness to use MEMS caps, willingness to provide informed consent

Exclusion Criteria:

• Treatment with any medication that is contraindicated in combination with ritonavir or its accompanying protease inhibitor

Subjects will be recruited from the Montefiore Medical Center Infectious Diseases Clinic who meet eligibility criteria described below.