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Recruitment of inflammatory leucocytes to the airways may play a part in the pathogenesis of asthma. As dietary enrichment with fish oil lipids can suppress leucocyte function, the effect of these lipids on asthma control and neutrophil function was studied in 20 subjects with mild asthma. Twelve subjects received capsules containing 3.2 g of eicosapentaenoic acid and 2.2 g of docosahexaenoic acid daily and eight subjects received placebo capsules containing olive oil for 10 weeks in a double blind fashion. Baseline specific airways conductance, airways responsiveness to histamine and exercise, diurnal peak expiratory flow, symptom scores, and bronchodilator use were measured. Neutrophil fatty acid composition was evaluated by gas chromatography, calcium ionophore induced neutrophil leukotriene (LT)B4 and LTB5 generation were measured by reverse phase high performance liquid chromatography and radioimmunoassay, and neutrophil chemotactic responses to formyl-methionyl-leucyl-phenylalanine (FMLP) and LTB4 were assessed by a microchemotaxis technique. Although the fish oil supplemented diet produced a greater than 10 fold increase in the eicosapentaenoic acid content of neutrophil phospholipids, there was no significant change in airways responsiveness to histamine or any change in any of the clinical measurements. After dietary supplementation with fish oil there was a 50% inhibition of total LTB (LTB4 + LTB5) generation by ionophore stimulated neutrophils and neutrophil chemotaxis was substantially suppressed. Neutrophil function remained unchanged in the placebo group. It is concluded that in subjects with mild asthma a fish oil enriched diet attenuates neutrophil function without changing the severity of asthma.