Seamen constitute a special group of international travellers who may run an increased risk of contracting hepatitis, because of visits to foreign ports and the particular environment on board ship. The purpose of the survey was to assess the prevalence of serological markers for hepatitis A, B and C virus infection among seamen and to identify present and previous risk factors for infection. 515 seamen were studied. The prevalence of antibodies against hepatitis A was 0.3% in subjects below 40 years of age, increasing with age above 40 years, and highest among those who had sailed in international trade. The prevalence of antibodies against hepatitis B was 2.7% in subjects below 40 years of age, increasing to 35.7% in the group above 60 years of age. Hepatitis C antibodies occurred in 1.2%. Vaccination of sailors against hepatitis A should follow the same recommendations as to other travellers. The prevalence of hepatitis B was higher than in reference groups of non-seamen but, because hepatitis B is only one of many blood-borne diseases, prevention should be directed towards changes in behaviour rather than vaccination, except for special groups. Young seamen in international trade were found to be most at risk of contracting sexually transmitted diseases.