The effect of a fish oil enriched diet containing about 3 g of eicosapentaenoic acid was studied in 10 patients with aspirin intolerant asthma. Subjects were studied during six weeks on a control diet followed by six weeks on the fish oil diet in a single blind study design. They were asked to record their peak expiratory flow (PEF) twice daily, bronchodilator and steroid doses, and subjective ratings of pulmonary symptoms on diary cards. There were no significant changes in symptom scores over the six weeks of either the control diet or the fish oil diet. PEF values, however, were significantly lower during the fifth and sixth week of the fish oil diet than during the control diet (308 v 262 l/min week 5 and 306 v 256 l/min week 6). Bronchodilator usage was also greater during the fifth and sixth week of the fish oil diet than during the control period (12.0 v 7.4 and 13.0 v 7.4 puffs a day in weeks 5 and 6). This pilot study suggests that fish diets may have a deleterious effect on patients with aspirin intolerant asthma.