The presence of antibodies (Abs) against hepatitis C virus (HCV) was analyzed in 26 haemophiliac patients; a prevalence of 38% was found in the total series and 56% in the treated patients. There were no cases with a positive serology among the three patients who had only received pasteurized factor. The frequency of detection of anti-HCV was higher in the patients who had a positive serology for HIV (67%) and hepatitis B virus (56%) than in the seronegative patients (24 and 29%, respectively). The patients with HCV Abs showed a decrease in the helper (CD4+) lymphocytic population, mainly due to the decrease in the helper inducer (CD4+/Leu8-) cells, such alterations being more evident in the patients who also were HIV+ who showed as well an increase in the T-cell activated lymphocytes (CD3+/la+) and a decrease in CD16+ natural killer cells. These results suggest that the lymphocytic alterations found in the patients with anti-HCV Abs are not specific and would rather be related to the presence or not of HIV infection.