The relationship between gender and mortality in nineteenth-century Europe has been highly debated. In particular, historians disagree about the manner and degree to which gender discrimination affected the mortality risk of the female population. This article contributes by examining the evidence of gendered mortality differences among children and adolescents in nineteenth-century Denmark. It makes use of both child sex ratios and mortality rates to explore the prevalence of excess female mortality. We show that the female mortality rate in Denmark was higher than that of males starting from around age four and lasting until adulthood, for the majority of the nineteenth century. This mortality gap, while initially narrow, was systematic and most pronounced in rural areas and during late adolescence. The gap was produced by a faster mortality decline among males. This pattern is clear both in time, as the gap widened during the nineteenth century, and during the life course, as the male mortality rate declined faster and reached lower levels during late childhood and early adolescence. While it is possible that various forms of gender discrimination slowed the mortality decline of females, the aggregated nature of the data limits our interpretation. However, by comparing the two mortality measures employed, we argue that in a low child-mortality setting such as Denmark, sex ratios are not always sensitive enough to measure excess female mortality in childhood. Further, since sex ratios primarily excel at measuring ‘hidden’ or unregistered mortality, they may be a suboptimal measure of mortality differences in the presence of a thorough and reliable vital registration system.