The evidence presented here favours the view that dietary supplementation with n-3 fatty acids results in attenuated activity of pro-inflammatory leukotrienes formed through the 5-lipoxygenase pathway in leucocytes. Neutrophil and monocyte chemotaxis seem to decrease, although this may depend on the patient groups studied. n-3 fatty acids increase the proliferative responses of lymphocytes. It is less clear, whether n-3 fatty acids affect other parts of leucocyte function. The possible value of n-3 PUFAs in inflammatory diseases in humans needs to be documented in carefully conducted clinical trials, which also should look carefully for possible side effects. However, the results obtained so far have been promising. The modulating effect of n-3 fatty acids on leucocyte activity-together with their other effects-could be favourable in atherosclerosis and coronary disease.