Hyperemesis gravidarum (hyperemesis), characterised by severe nausea and vomiting in early pregnancy, has an unknown aetiology. The aim of the present study was to investigate food and nutrient intake before pregnancy and the risk of developing hyperemesis in women participating in the Norwegian Mother and Child Cohort Study. From 1999 to 2002, a total of 7710 pregnant women answered a FFQ about their diet during the 12 months before becoming pregnant and a questionnaire about illnesses during pregnancy, including hyperemesis. Only women who were hospitalised for hyperemesis were included as cases. Nutrient intakes during the year before pregnancy did not differ between the ninety-nine women who developed hyperemesis and the 7611 who did not. However, the intake of seafood, allium vegetables and water was significantly lower among women who developed hyperemesis than among women in the non-hyperemesis group. Relative risks of hyperemesis were approximated as OR, and confounder control was performed with multiple logistic regression. Women in the upper tertile of seafood consumption had a lower risk of developing hyperemesis than those in the lower tertile (OR 0·56, 95 % CI 0·32, 0·98), and women in the second tertile of water intake had a lower risk of developing hyperemesis than those in the first tertile (OR 0·43, 95 % CI 0·25, 0·73). The findings suggest that a moderate intake of water and adherence to a healthy diet that includes vegetables and fish are associated with a lower risk of developing hyperemesis.