In a pilot study six patients with active ulcerative colitis and six healthy controls were given fish oil (MaxEPA) containing 3-4 g of eicosapentaenoic acid daily for a period of 12 weeks. There was a significant improvement in the patients' symptoms and histological appearance of the rectal mucosa by the end of the treatment period. There was significant fall in neutrophil chemiluminescence during treatment in patients, whereas no change was observed in the control group. Neutrophil leukotriene B4 levels fell significantly during treatment. Serum from patients receiving fish oil was significantly less chemotactic for neutrophils compared with control serum. Eicosapentaenoic acid inhibited neutrophil chemotaxis and chemiluminescence in vitro. The omega-3 fatty acids, which occur naturally in fish oils, may exert a beneficial effect by decreasing the production of inflammatory mediators.