Prostitutes may be regarded as the reservoir of sexual transmitted diseases and their customers as the transmitters. The use of condom for sexual intercourse with prostitutes is an effective way of preventing sexually transmitted infections. Deep sea fishermen usually spend a long period of time sailing the oceans, and many have contacts with prostitutes when their boat stays in harbor. This is a study of the factors which affect the fishermen who are customers of prostitutes and condom using. In all 859 Taiwanese deep sea fishermen were interviewed and samples of their blood serum were collected between January and June of 1991. Of the interviewees, 628 (73.1%) used at one time the services of a prostitute; 84% of aborigines from reserved areas and 68% of aborigines from non-reserved areas. The factors which were found to be related to a sailor's tendency to become the customer of a prostitute after adjustments in logistic regressions were marital status, educational level, tobacco smoking and alcohol consumption. Of the 628 men 100 claimed to use condoms always, 110 sometimes and the remaining 414 men never. The vast majority of aborigines from reserved areas (113/124; 91.1%) never used condoms as compared with 84.0% of aborigines from non-reserved areas and 81.8% of non-aborigines (included Hakka). As might be expected, the lesser educate as well as the aborigines from reserved areas use condoms the least frequently.