Both parity and a young age at first pregnancy are associated with a reduction in breast cancer risk. The hormones involved in this process are not fully investigated. Human chorionic gonadotropin is a placental hormone, which in rats and in human breast cells in vitro has been shown to prevent against breast cancer. Hyperemesis, a severe nausea combined with vomiting during pregnancy, is associated with increased levels of human chorionic gonadotropin. We investigated the possible relationship between hyperemesis and subsequent breast cancer risk in a case-control study based on registry data. Among 13 079 breast cancer cases and 34 348 individually matched controls we found 148 cases and 405 controls who had been hospitalised for hyperemesis. Hyperemesis was not associated with breast cancer risk (adjusted odds ratio 1.05, 95% confidence interval 0.86-1.27), and similar risks were observed regardless of age at diagnosis, number of hospitalisations for hyperemesis or time of follow-up. Our results do not support the hypothesis that human chorionic gonadotropin is responsible for the protective effect of pregnancies upon breast cancer risk.