The objective of the present study was to assess the impact of baseline thymic volume on the CD4 cell repopulation induced by highly active antiretroviral therapy (HAART) in HIV-infected adults. Therefore, 37 previously untreated HIV-1-infected adults were prospectively followed from August 1998 to September 2001. All patients underwent a thorax CT scan at starting HAART to measure thymic volume. The median follow-up time was 189 [87.5-498.5] days. CD4 cell repopulation was considered as an increase > or = 200 cells/mm3 above baseline count. Twenty-one (57%) patients achieved this CD4 repopulation. Baseline thymic volume was the main independent factor associated with CD4 repopulation (P = 0.016); this factor was a positive predictor (adjusted hazard ratio: 1.25 [95% confidence interval 1.1-1.4]). Although, CD4 cell count and non-AIDS diagnosis at baseline were associated with CD4 cell repopulation on the univariate analysis (P = 0.03 and P = 0.02, respectively), no statistical signification was found on the multivariate test. In summary, baseline thymic volume is a predictor of early CD4 cell repopulation in adult HIV-1-infected patients taking HAART.