Nonadherence to highly active antiretroviral therapy (HAART) is a major cause of human immunodeficiency virus (HIV) drug resistance; however the level of nonadherence associated with the greatest risk of resistance is unknown. Beginning in February 2000, 195 patients at the Johns Hopkins Outpatient Center (Baltimore, MD) who were receiving HAART and who had HIV loads of <500 copies/mL were recruited into a cohort study and observed for 1 year. At each visit, adherence to HAART was assessed and plasma samples were obtained and stored for resistance testing, if indicated. The overall incidence of viral rebound with clinically significant resistance was 14.5 cases per 100 person-years. By multivariate Cox proportional hazards regression, a cumulative adherence of 70%-89%, a CD4 cell nadir of <200 cells/microL, and the missing of a scheduled clinic visit in the past month were independently associated with an increased hazard of viral rebound with clinically significant resistance. Clinicians and patients must set high adherence goals to avoid the development of resistance.