We prospectively studied side effects about 54 patients with chronic hepatitis C treated with 3 to 10 MIU a day of interferon (IFN) alpha, which was administrated for 16 to 24 weeks. Every day, all of them wrote down every symptoms, by themselves, during its treatment. Any symptoms occurred in all patients and each incidence of symptoms such as fever, fatigue, headache, anorexia, arthalgia, myalgia, chill, itching, insomnia, nausea, numbness of hand and foot, irritability, diarrhea, eye ball pain, vomiting, were all higher than those which have been reported by some papers in Japan. So, it was considered that the symptom self-wrighting method by patient was useful to evaluate the entity of side effects. Furthermore, we studied 26 patients, who discontinued IFN treatment because of side effects and analyzed the background factors. Each incidence of symptoms of these patients were not always compatible to those incidences. But by observation of those symptoms, we could know severe side effects earlier.