Regulations implemented by World Athletics (WA) require female athletes with differences of sexual development to suppress their blood testosterone levels in order to participate in certain women’s sporting competitions. These regulations have been justified by reference to fairness. In this paper, we reconstruct WA’s understanding of fairness, which requires a “level playing field” where no athlete should have a significant performance advantage based on factors other than talent, dedication, and hard work over an average athlete in their category. We demonstrate (...) that by placing regulations only on testosterone levels, while ignoring physical as well as socioeconomic advantages, WA consistently fails to meet its own definition of fairness. We then discuss several ways in which this definition could be met. Our analysis shows that a categorical system, in which athletes are divided into categories based on properties leading to significant performance advantages, is best suited for meeting WA’s definition of fairness. (shrink)