A 62-year-old man with chronic hepatitis C underwent interferon (IFN)-β therapy. After treatment for a period comprising 29 months and 2 weeks, hematological results showed a decrease in white blood cell, hemoglobin, and platelet counts (WBC 2,300/µl, Hb 7.2 g/dl, PLT 4.7×10(4)/µl), and IFN therapy was stopped. Despite therapy discontinuation, the pancytopenia continued to progress with elevation of LDH (LDH 4,898 IU/l), and the patient was admitted to our hospital with suspected hematological disease. The patient underwent clinical screening, and pernicious anemia caused by vitamin B12 deficiency was diagnosed. The anemia rapidly improved with vitamin B12 treatment. Interferon is the mainstay of treatment for patients with viral hepatitis. While the adverse effects of interferon therapy are widely recognized, only a few reports have documented pernicious anemia developing during IFN-therapy. We recommend that particular attention be paid to such clinical and laboratory conditions as megaloblastic anemia when administering IFN. We also recommend checking the vitamin B12 level, as a deficiency of this vitamin may lead to the development of megaloblastic anemia.