Transaminase elevation is frequently seen in hepatitis C virus (HCV)-HIV-coinfected patients receiving antiretroviral therapy (ART), representing an increase in the immune response against HCV and being one of the mechanisms proposed to be involved. There is a report claiming that HCV genotype 3 is an independent risk factor. Our objectives were to assess the incidence of liver toxicity in an HIV-HCV-coinfected population with relatively preserved cellular immunity, and the role of HCV genotypes in the elevation of liver enzymes, both at baseline and after initiating ART. All HIV(+) patients with positive anti-HCV serology and CD4(+) cell counts above 100/mm(3) who began triple ART were identified, and their HCV-RNA levels and HCV genotype were determined. Liver enzymes were determined at baseline and bimonthly during follow-up. Of anti-HCV patients 147 were included, 128 (87.1%) of whom had detectable plasma HCV-RNA. HCV-1 and HCV-4 genotypes were found to confer an increased probability of having at baseline transaminases within normal limits over the other genotypes. Severe transaminase elevations (grades 3 and 4) occurred in 5/124 patients (4.0%), all with high pre-HAART ALT and positive HCV-RNA levels. Multivariate analysis showed that patients with genotype HCV-3 had a 3.27 times higher risk of developing HAART-related transaminase elevations of any grade. In conclusion, subjects with the HCV-1 genotype more often had transaminases within normal limits at baseline. The incidence of severe transaminase elevation after initiating ART was very low (4%) in this HIV(+) population with relatively preserved cellular immunity. HCV genotype 3 was identified as a risk factor for the development of transaminase elevation of any grade.