A retrospective cohort study was performed to analyze the mortality patterns of Danish merchant seamen in the period 1970-1985. The population census in 1970 in Denmark was the source of information on individual occupation, age, and marital status. All men aged 20-64 years and economically active in 1970 were included. Computerized linkage with the Danish Mortality Register gave information about the deceased persons' date and cause of death. An increased overall mortality among all groups of seamen was found, being highest for deck and engine crew members. The overall mortality was strongly dependent on age and marital status. The highest mortality rate ratios (MRR) were found among young seamen and unmarried seamen. MRRs of 1.90 and 2.47 for cancer of the respiratory system were found among engine officers and crew. The MRRs for accidents and suicide were increased for all seamen, and were highest for crew members, among whom the MRR from accidents was stable within age groups but fell for suicide with increasing age. The same pattern was found with cirrhosis of the liver, although this was positively associated with increasing age. Excess mortality from ischemic heart disease was only found among engine crew (MRR = 1.43). This study confirms earlier findings of high mortality among seafarers. Negative selection into the occupation, occupational environmental factors, and lack of health and safety promotion programs and education could be causes of the high mortality.