This article examines the recent rise in the sex ratio at birth in Vietnam and relates its emergence to kinship systems and ethnic composition using 2009 census micro-data. Presentation of the main socioeconomic and ethnic differentials in birth masculinity is followed by a review of the three intermediate factors leading to increases in the sex ratio at birth: prenatal technology, declining fertility, and gender bias. An indirect measurement of fertility behavior is used to demonstrate the close association between levels of the sex ratio at birth and the intensity of son preference. Data on household composition indicate that Vietnam is characterized by the co-existence of kinship patterns typical of East and Southeast Asia. Son preference in Vietnam is found to be related to the prevalence of more traditional patrilineal systems. The article concludes by considering the implications of the cultural dimensions of prenatal sex selection for policy responses and for the likely future change in the sex ratio at birth.