It was not until 1943, amid world war, that penicillin was found to be an effective treatment for syphilis. This study investigated the hypothesis that a decrease in the cost of syphilis due to penicillin spurred an increase in risky non-traditional sex. Using nationally comprehensive vital statistics, this study found evidence that the era of modern sexuality originated in the mid to late 1950s. Measures of risky non-traditional sexual behavior began to rise during this period. These trends appeared to coincide with the collapse of the syphilis epidemic. Syphilis incidence reached an all-time low in 1957 and syphilis deaths fell rapidly during the 1940s and early 1950s. Regression analysis demonstrated that most measures of sexual behavior significantly increased immediately following the collapse of syphilis and most measures were significantly associated with the syphilis death rate. Together, the findings supported the notion that the discovery of penicillin decreased the cost of syphilis and thereby played an important role in shaping modern sexuality.