The aim of this study was to examine whether hepatitis C virus (HCV) pretreatment quasispecies complexity was linked to virological response or other clinical and biological parameters, in human immunodeficiency virus (HIV)-coinfected patients undergoing anti-HCV treatment. In addition, HCV quasispecies composition is described longitudinally in these patients before, during, and after treatment. The 52 HIV-coinfected patients were included in a randomized therapeutic trial. At inclusion, they had CD4(+) counts of >250/micro l, HIV plasma load of <10,000 copies/ml, and chronic HCV infection with genotype 1 (n = 27), 2 (n = 2) or 3 (n = 23). These values were compared at baseline with 32 HCV-only-infected, interferon-naive patients who were infected with genotype 1, 2, or 3 (n = 16, 1, or 15, respectively). HCV complexity was studied by single-strand conformation polymorphism (SSCP) in E2 hypervariable region 1 (HVR1), and diversity was evaluated at inclusion in 20 coinfected patients by sequencing four major SSCP bands. The baseline number of SSCP bands was identical in HIV-infected and control patients. In HIV-infected patients, HCV complexity was not predictive of sustained virological response to anti-HCV treatment and was unrelated to epidemiological factors, immunological parameters linked to HIV infection (CD4(+) counts, T CD4(+) proliferative responses to HIV-1 p24), protease inhibitor treatment, HCV plasma load, or genotype. HCV diversity was lower in genotype 2- and 3-infected patients. Six months after completion of the anti-HCV treatment, in comparison with baseline, SSCP profiles were modified in 13 of the 21 nonresponding coinfected patients with analyzable samples. In conclusion, in HIV-infected patients, HCV variability had no significant influence on virological response to anti-HCV treatment.