Objective: To determine the impact of age and initial HAART regimen class on virologic and immunologic response within 24 months after initiation. Design: Pooled analysis of data from 19 prospective cohort studies in the North American AIDS Cohort Collaboration on Research and Design (NA-ACCORD). Methods: Twelve thousand, one hundred and ninety-six antiretroviral-naive adults who initiated HAART between 1998 and 2008 using a boosted protease inhibitor-based regimen or a nonnucleoside reverse transcriptase inhibitor (NNRTI)-based regimen were included in our study. Discrete time-to-event models estimated adjusted hazard odds ratios (aHOR) and 95% confidence intervals (CIs) for suppressed viral load (≤500 copies/ml) and, separately, at least 100cells/μl increase in CD4 cell count. Truncated, stabilized inverse probability weights accounted for selection biases from discontinuation of initial regimen class. Results: Among 12 196 eligible participants (mean age = 42 years), 50% changed regimen classes after initiation (57 and 48% of whom initiated protease inhibitor and NNRTI-based regimens, respectively). Mean CD4 cell count at initiation was similar by age. Virologic response to treatment was less likely in those initiating using a boosted protease inhibitor [aHOR = 0.77(0.73, 0.82)], regardless of age. Immunologic response decreased with increasing age [18-<30: ref;30-<40: aHOR = 0.92 (0.85, 1.00); 40-<50: aHOR = 0.85 (0.78, 0.92); 50-<60: aHOR = 0.82 (0.74, 0.90);>60: aHOR = 0.74 (0.65, 0.85)], regardless of initial regimen. Conclusion: We found no evidence of an interaction between age and initial anti- retroviral regimen on virologic or immunologic response to HAART; however, decreased immunologic response with increasing age may have implications for age-specific when-to-start guidelines. © 2010 Wolters Kluwer Health | Lippincott Williams & Wilkins.