We examined the risk and determinants of developing severe liver toxicity in 108 HIV-infected patients showing adherence to nevirapine- and efavirenz-containing regimens. Between January 1997 and December 2000, 70 patients were treated with nevirapine- and 38 patients with efavirenz-containing regimens, for a median period of 127 days (interquartile range 65-240). Severe liver toxicity was defined as grade 3-4 elevations (>5 x upper limit of normal) of aminotransferases AST or ALT. A total of 22 (20%) patients showed severe liver toxicity, 17 of them were treated with nevirapine- and five with efavirenz-containing regimens (relative risk [RR]: 1.85, 95% confidence intervals [CIs] 0.74-4.61; P=not significant). Multivariate analysis showed the association of severe liver toxicity with hepatitis C antibody positive (RR 7.64; 95% CI: 1.48-39.52; P=0.01), nevirapine- or efavirenz-containing regimens combined with a protease inhibitor (RR: 3.07, 95% CI: 1.01-9.32, P=0.04) and alcohol intake greater than 40 g per day (RR: 3.09, 95% CI: 1.27-7.54, P=0.01). These findings have potential implications for selecting and monitoring antiretroviral therapy in HIV-infected patients with hepatitis C virus coinfection and for avoiding alcohol intake during antiretroviral therapy.