A prospective study to evaluate the incidence of herpes zoster (HZ) as an immune restoration disease in patients with AIDS during highly active antiretroviral therapy (HAART) was conducted in a series of 115 patients diagnosed with AIDS initiated on HAART between 1 January 2000 and 31 July 2001. Of these, a single dermatomal HZ episode occurred in 14 (12%) patients within one and 15 months of HAART (median eight months). The HZ patients were similar to the non-HZ patients in age, sex, and HIV transmission risk factor, but had a more advanced disease. Compared with the baseline values, the viral loads significantly (P<0.01) decreased, while the mean CD4+ T-cell counts increased by almost four-fold (P<0.01) in both groups at the time of the HZ episode (or equivalent in non-HZ), but remained below 400/mL in the HZ patients. HZ during HAART is an immunopathological consequence of the improvement of the host immuneresponse, correlating with the beginning of immune restoration.