In a cohort of 128 human immunodeficiency virus-infected patients, we found that patients' knowledge of antiretroviral dosing was suboptimal at regimen initiation but improved with time. Poor medication knowledge 8 weeks after regimen initiation was associated with lower adherence and with a lower level of literacy in a multivariate model (P=.03). Because knowledge deficits are common after antiretroviral regimen initiation, clinicians should assess patients' understanding of medication dosing soon after regimen initiation or change.