Sexually transmitted infections lead to considerable costs and problems from a socioeconomic perspective, as well as in terms of patient health. In order to effectively manage sexually transmitted infections, it is necessary to establish basic epidemiologic data in this field. This study investigated prevalence rates of syphilis, gonorrhea, and chlamydia among 1612 persons aged 20-59 years among the general population who visited health examination centers in Korea, and examined the sexual behavior of young and middle-aged adults using a questionnaire survey. In total, 807 males and 805 females participated in this study, and among the 1612 subjects, 6, 7, and 90 were gave positive results in syphilis, gonorrhea, and chlamydia infection tests, respectively, indicating prevalence rates of 0.37, 0.43, and 5.58%. In the questionnaire survey, 85.8% of the respondents answered that they had not taken any test for a sexually transmitted infection, and chlamydia-positive respondents were observed to have had a significantly higher number of sexual partners. Among all of the respondents, 43.7% used a condom only sometimes during sexual intercourse, while 15% did not usually do so. Future studies to follow up the incidence and prevalence rates of sexually transmitted infections in detail in young adults and middle-aged people are necessary, and the results of this study will be utilized as basic data for them.