We have experienced ten cases of hepatitis, type A, from March to April in 1983. All patients have taken raw oyster before hepatitis developed. The symptoms such as fever and/or diarrhea developed within 4 weeks after the patients have taken oyster. No cases of hepatitis in children were found out. From these results, it was concluded that the causes of hepatitis, type A, which we have experienced, are associated with raw oyster. HAV was not detected in shellfish by the method of radioimmunoassay.