Dietary fat consumption is hypothesized to influence atopy development by modulation of IgE production. The aim of our study was to assess whether margarine consumption is associated with allergic sensitization and diseases in children. Data of a cross-sectional health survey in 1998-1999 comprising 2,348 children age 5 to 14 yr were analyzed. Information on type of fat used as spread during the past 12 mo, children's health, and sociodemographic factors were gathered by questionnaire. Allergic sensitization to common aeroallergens was assessed by specific serum IgE. Compared with butter consumption, margarine consumption was associated with allergic sensitization (adjusted odds ratio 1.30 [95% confidence interval: 1.01 to 1.67]) and with rhinitis symptoms during the past 12 mo (1.41 [1.01 to 1.97]). Sex-stratified analysis showed that these associations were limited to boys (boys: sensitization 1.57 [1.12 to 2.20], rhinitis symptoms 1.76 [1.12 to 2.78]; girls: sensitization 0.99 [0.67 to 1.46], rhinitis symptoms 1.03 [0.63 to 1.70]). No statistically significant relation was observed between exclusive margarine consumption and ever physician-diagnosed hay fever or asthma in all children. In conclusion, the sex difference in the association of margarine consumption with allergic sensitization was in accordance with the higher IgE concentrations and atopy prevalence in boys compared with girls. Increased intake of certain polyunsaturated fatty acids might further stimulate IgE production in boys.