Race is conspicuously absent from this story. The majority of cases studies in the first half of the twentieth century focused on white patients, and medical theories about intersexuality did not address racial difference directly. Racial differentiation nevertheless underwrote investigations in pediatric endocrinology, and debates about sexual difference were structured by racialized conceptions of genitalia and sexuality. At the same time, new and interrelated social theories postulated that race and sex were cultural rather than biological categories. This paper explores the tension between the emergence of "cultural fluidity" as a concept and the endurance of biological difference in medical practice. Using medical case studies, I explore how practitioners explained the predominance of white patients, how conceptions of racial difference structured the medical analysis of the few cases of African American patients, and how racial segregation shaped debates about symptoms such as pigmentation and skin-darkening in white intersex patients. I argue that including race into the historical analysis of the medicalization of sex highlights the tension between the insistence on racial and sexual difference in medical practice and the acknowledgement of the socio-cultural nature of race and sex.