Guidelines recommendation to extend treatment duration in genotype 1 hepatitis C virus (HCV)/HIV-coinfected patients who clear the virus later than treatment week 4 is not evidence-based. Our main objective was to study the ability of week 12 viral response early virologic response (EVR) to predict long-term outcome in patients treated for 48 weeks. Multicenter retrospective cohort analysis. Genotype 1 HCV treatment-naive, HIV-coinfected adult patients with compensated liver disease who started combination therapy with fixed-dose pegylated-interferon (pegIFN) alfa-2a or weight-based pegIFN alfa-2b plus ribavirin were included. Univariate and forward stepwise logistic regression analysis were used to identify predictors of sustained viral response (SVR) and relapse. By intention-to-treat analysis, 31.3% (87/278) of patients achieved an SVR. SVR rate was more than three-fold higher in patients who cleared the virus by week 12 of treatment compared with late responders. Among 123 end-of-treatment responders, 36 (29.3%) relapsed. Relapse risk increased in patients with cirrhosis, in those with ribavirin dose reductions and in late responders: more than 65% of patients who cleared the virus between weeks 12 and 24 relapsed following 48 weeks of treatment compared with 10% of those attaining a complete EVR (<15 IU/ml) at treatment week 12 (risk ratio 6.4, 95% confidence interval 2.9-14.4). Viral response at treatment week 12 is a strong predictor of long-term outcome. Genotype 1 HCV/HIV-coinfected patients who achieve a complete EVR (<15 IU/ml) are at low risk of viral relapse after completing the standard 48 weeks of therapy.