A prospective follow-up study of the incidence of HIV-1 seroconversion was performed among young men who were inducted into military service in northern Thailand during 1991. The 1,932 HIV-1 seronegative men in the study were followed until their discharge from the military two years later. Several possible risk factors related to HIV-1 seroconversion among these military conscripts were examined. Among young men who visited a commercial sex worker (CSW), 1,698 person-years of follow-up were observed; among young men who did not visit a CSW, 1,731 person-years were observed. A total of 81 men became HIV-1 infected during the study; 77.8% of these cases had visited a CSW. 1. set up a 2*2 table for these data, using visiting a CSW as the exposure (5 points)