BACKGROUND: The unique health needs of a growing older adult population infected with human immunodeficiency virus (HIV) require study, especially in terms of the response to and tolerability of highly active antiretroviral therapy (HAART). METHODS: Changes in HIV clinical markers after HAART initiation were compared among 2259 patients aged 18 to 39 years (reference group), 1834 patients aged 40 to 49 years, and 997 patients 50 years or older enrolled in an integrated health care system. RESULTS: Patients 50 years or older were more likely to achieve HIV RNA levels of less than 500 copies/mL within 1 year of HAART initiation (hazard ratio [HR], 1.15; P =.009), but adjustment for adherence attenuated this finding (HR, 1.03; P =.59). Subsequent HIV RNA level rebound (to > or =1000 copies/mL) was less likely among patients aged 40 to 49 years (HR, 0.81; P =.01), which persisted after adjustment for adherence (HR, 0.79; P =.004). In year 1 of HAART, younger patients had larger CD4 T-cell count increases (131.8, 121.3, and 111.8 CD4 T cells/microL per year among patients aged 18-39, 40-49, and > or =50 years, respectively; P =.046). In years 2 through 6, older patients had larger CD4 T-cell count increases (4.5, 11.6, and 9.7 CD4 T cells/microL per year among patients aged 18-39, 40-49, and > or =50 years, respectively; P =.04). After adjustment for adherence, age differences in CD4 T-cell count changes remained in year 1 (P =.02) but not in years 2 through 6 (P =.08). Other factors, including comorbidities, had no effect on study results. Metabolic (glucose and lipids), hematologic (absolute neutrophils and hemoglobin), and renal (creatinine) abnormalities were more likely among older patients. CONCLUSION: Despite a higher risk of adverse events, patients 50 years or older sustained high therapy adherence to maintain improved virological outcomes and to compensate for their early blunted CD4 T-cell count response compared with younger patients.