The sick girl was a popular stereotype in Swedish medical discourse around 1900. It was established by medical authorities at the time that a substantial number of Swedish girls suffered from various diseases and ailments. However, at the beginning of the twentieth century, at a time when the welfare state was gradually evolving in the Nordic countries, the scientific opinion of girls changed. The new girl was represented as healthy and active. This article examines the medical discourse on girls, and their activity and health in Sweden during circa 1880 to 1930. It reveals patterns of the medicalization of girls as well as categorizations and constructions of girlhood that corresponded with contemporaneous notions of gender. It reveals a recurring, if inconstant, problematization of girls' illness and lack of adequate physical activity. In this article I will show how the discussions about girls around 1900 share several similarities with current ones.