To determine the predictive value of IgM anti-hepatitis C virus (HCV) testing in patients with chronic hepatitis C infections undergoing interferon-alpha (IFN-alpha) therapy, IgM anti-HCV reactivity was analysed by two different methods (non-commercial and commercial) in 19 patients and monitored at times 0 (pretreatment), 3, 6, 12, and 24 months during follow-up. Eight patients were non-responders, five remained in sustained response 1 year after stopping treatment, and six had a relapse. No correlation between alanine transaminase (ALT) levels and IgM anti-HCV reactivity was found by either method in baseline samples. In addition, neither the presence nor absence of IgM anti-HCV in baseline samples, nor the loss of specific IgM reactivity during treatment, had any predictive value. Finally, no other parameters analysed (age, sex, risk group and histological diagnosis), were significantly associated with IgM anti-HCV reactivity in our study. In summary, these results suggest that baseline detection and monitoring of IgM anti-HCV reactivity are not useful in predicting the sustained response to IFN-alpha therapy in chronic hepatitis C infection.