A new socioeconomic index scored for census tract characteristics (income, education, and household crowding) was used for study of the socioeconomic patterns of incidences of syphilis, gonorrhea, and squamous cell carcinoma of the uterine cervix in Oregon's three metropolitan statistical areas. Cases of syphilis and gonorrhea were reported, but cases of uterine cancer were found by intensive searches of records from hospitals and pathology laboratories. For all three diseases there was a striking inverse relationship between incidence and urban socioeconomic status, a fact that indicated that this distribution pattern is probably real and not a function of reporting bias. This strong socioeconomic effect on the incidence of sexually transmitted diseases probably reflects corresponding differences in frequencies of nonmarital sexual contacts among the different socioeconomic strata; these differences should be recognized in our efforts to develop more rational control measure for application in the community. From the information available about contacts of persons with syphilis, it appeared that this socioeconomic incidence pattern was a function of heterosexual behavior.