Hepatocellular carcinoma (HCC) is one of the most common cancers worldwide. However, HCC is rare in young Japanese patients and the clinical features of young patients with HCC have not yet been fully studied. This study was designed to determine the clinical characteristics and prognosis of patients with HCC who are younger than aged 40 years. A retrospective analysis was performed for patients newly diagnosed with HCC and observed from January 1990 to December 2003 at our hospitals. Patients younger than aged 40 years at the diagnosis of HCC were defined as the young group and were reviewed. There were 20 patients (16 males) with HCC who were younger than aged 40 years. The mean age at diagnosis was 33.6 (range, 20-39) years. Fifteen of 20 patients were positive for hepatitis B surface antigen (HBsAg) and 2 patients were positive for hepatitis C virus antibody. According to the Child-Pugh grading, the liver function was relatively good in all patients. Because most of the patients did not receive periodic follow-up, this disease often was discovered at an advanced stage, usually after the appearance of some symptoms. Although intensive treatment was performed for such young patients, the survival was nevertheless poor. Most patients died from this cancer within 1 year. However, one patient who received periodic follow-up and also was in relatively good physical condition had a better prognosis, and he survived for 88 months. Young patients with HCC tended to have a poor prognosis because of advanced stage of HCC, despite a well-preserved liver function and aggressive treatment. Screening for HCC and an early diagnosis is needed for such patients to demonstrate an improved prognosis, especially for HBsAg-positive patients.