The hepatitis C virus (HCV) infection is associated with a wide spectrum of clinical entities ranging from asymptomatic carriage to severe forms of chronic hepatitis. In Egypt, HCV infection has been shown to be highly prevalent. The aim of this study was to determine the frequency and significance of anti-HCV IgM in the sera of clinically healthy blood donors and chronic HCV patients, whose sera were also positive for anti-HCV IgG. Anti-HCV IgM was detected in the sera of 7 (46%) of the blood donors (n = 15), of whom 5 (71%) had a positive HCV-RNA. The corresponding results in patients with a chronic hepatitis C (CHC) infection (n = 19) were 8 (42%) and 5 (62%) respectively. The detection of anti-HCV IgM did not correlate with a positive test for HCV-RNA (R = 0.2) in the CHC patients. However, the levels of anti-HCV IgM in CHC patients were associated significantly with the level of serum transaminase, a finding that can be used in monitoring disease activity in such a group of patients. On the other hand, a significant association was evident between the detection of anti-HCV IgM and HCV-RNA in the sera of blood donors. Thus among the blood donors, viraemia correlates well with the detection of HCV-IgM Ab, but it cannot be excluded in its absence. The presence of HCV-IgM in some patients with CHC infection indicates that the antibody as a viral marker may not be unique to acute HCV infection.