To assess incidence, risk factors, histology, and outcome of severe hepatotoxicity (SH) during antiretroviral treatment (ART). Seven hundred fifty-five HIV-seropositive patients consecutively prescribed new ART were selected. Liver function tests were assessed at baseline, after 1 month, and every 4 months thereafter. Liver biopsy was recommended in case of SH (i.e., increase in liver enzymes >/=10 times the upper limit of normal or 5 times baseline if markedly abnormal). Twenty-six cases of SH were observed with an incidence of 4.2% person-years. Liver failure (LF) was rarely seen (1.1 per 100 person-years). Liver damage was invariably observed in patients with chronic viral hepatitis. Liver histology showed exacerbation of viral hepatitis in all 16 patients for whom a liver biopsy was available at the time of SH. A direct correlation was found between alanine aminotransferase increase and increase in CD4 T-cell count in patients with SH (r = 0.53, p <.001). Death occurred during follow-up in 7 of 26 (27%) patients, all of whom showed LF and baseline CD4+ count less than 200 cells/mm(3) (7/7 patients = 100% vs. 8/19 patients without LF; p <.01). Relapse of SH was observed after ART was recommenced in 7 of 17 (41%) patients. Five of these 7 patients did not show further SH relapse after treatment with interferon. This study provides estimates of SH and LF in a large population-based setting where hepatitis C virus coinfection is highly prevalent and provides indications that liver damage may be caused by immune reconstitution and related exacerbation of viral hepatitis. A strict follow-up for hepatotoxicity is mandatory when ART is initiated in patients with <200 CD4+ T cells/mm(3). Antihepatitis pre- or comedication could be an effective preventive or curative measure.