OBJECTIVE Alpha-interferon therapy may occasionally account for immune-mediated phenomena. This study was conducted in an attempt to investigate the incidence of the development of immune-mediated dermatological diseases during alpha-interferon therapy in patients with chronic viral hepatitis. The latter has not been evaluated prospectively, whereas most of the previous studies examined small numbers of interferon treated patients or consisted of case reports. METHODS A prospective case-control study. METHODS A tertiary referral centre. METHODS One hundred and twenty consecutive patients with chronic viral hepatitis (67 with hepatitis B, 45 with hepatitis C, six with both hepatitis viruses, and two with delta hepatitis) were evaluated during a course of alpha-interferon therapy. In addition, 120 consecutive patients with chronic liver diseases (disease control group), who had never received alpha-interferon therapy, were evaluated during the period of the study (at least for 12 months). METHODS Recombinant alpha-interferon at a dose of 4.5 or 5 million units subcutaneously (s.c.) three times per week for 6 to 12 months was administered to patients with hepatitis B. The patients with chronic hepatitis C were treated with 3 million units s.c. three times per week for 12 to 18 months. The patients with chronic hepatitis B and C infections received 4.5 million units for 6 months, and then 3 million units for an additional 6 to 12 months. Finally, the patients with chronic delta hepatitis received 5 million units for 1 year or more. METHODS To assess prospectively the incidence of these dermatological disorders during alpha-interferon therapy and to estimate if there is any relationship between their development and the clinical, laboratory or other characteristics of the patients with chronic hepatitis. RESULTS Three to 6 months after the initiation of alpha-interferon three patients with chronic viral hepatitis (two with hepatitis C and one with hepatitis B) developed lichen planus, whereas one patient with hepatitis C developed relapsing aphthous stomatitis. The development of these disorders was significantly associated only with the presence of antinuclear antibodies before the initiation of alpha-interferon (P=0.000000). None of the patients from the disease control group had such a manifestation during the follow-up. Lichen planus resolved after the end of therapy in all of them. In contrast, therapy was discontinued in the patient who developed aphthous stomatitis, owing to the painful lesions. CONCLUSIONS This study demonstrated that alpha-interferon may rarely (3.3%) induce immune-mediated dermatological disorders, especially lichen planus. The development of these disorders may reflect a subclinical or covert autoimmune background of patients, as suggested by the presence, although in low titres, of antinuclear antibodies. However, when lichen planus developed, it was mild, did not require the discontinuation of therapy and resolved after alpha-interferon administration had ceased.