Twenty healthy, non-smoking subjects were enrolled into a study to investigate the effects of dietary supplementation with essential fatty acid (EFAs) on red blood cell rheology. Ten subjects were given 3 months dietary supplementation with long chain polyunsaturated EFAs containing omega-3 and omega-6 EFAs while 10 others were given placebo (sunflower oil). Venous sampling was performed at 0 and 12 weeks and red blood cell (RBC) aggregation and deformability measured by a filtration system. The results showed a reduction in RBC aggregation in the group given omega-3 and omega-6 EFAs but not in the placebo group. This may be related to changes in the RBC membrane and surface receptor characteristics. Such EFAs may be useful in Raynaud's phenomenon.