The dietary intake of certain fatty acids might contribute to the development of allergic diseases such as hay fever and asthma. We investigated the as soci- ation between the concentrations of fifteen fatty acids in serum phospholipids, as a marker of dietary intake and metabolism, and hay fever, allergic s en- sitisation and total IgE in adults. Data from a population-based cross-sectional study on respiratory health, including the measurement of fatty ac ids in the serum phospholipids of 740 adults between 20 and 64 years of age, were analysed. Positive associations were found between hay fever and arachidonic ac id, and allergic sensitisation and oleic acid. No other fatty acids showed any association with hay fever or allergic sensitisation. Elevated levels of t otal IgE were not related to fatty acids. Concentrations of long-chain n -3 fatty acids, trans fatty acids or saturated fatty acids in serum phospholipids were not associated with allergic diseases in adults in this study. The present result on the association between hay fever and arachidonic acid is consistent with curren t hypoth- eses but warrants further research.