It is now widely accepted that dietary supplementation with fish or fish products reduces the risk of cardiovascular diseases. This beneficial effect has been attributed to the n-3 polyunsaturated fatty acid content (n-3 PUFA). Volunteers (190) from 3 European countries: Iceland.(78), Ireland (42) and Spain (70), were included in 3 controlled diets with 30% caloric restriction, one with lean fish (cod), one with fatty fish (salmon), and another without fish but with high oleic sunflower oil capsules. Salmon diet was responsible for a statistical significant increase of total n-3 fatty acids, particularly EPA (eicosapentaenoic acid, 20:5n-3) and DHA (docosahexaenoic acid, 22:6n-3, p < 0.001) in erythrocytes membrane fatty acids prepared from volunteers' blood. On the other hand, total n-6 fatty acids decreased significantly (p < 0.001) and AA (arachidonic acid, p = 0.021) was mainly responsible for this result. Cod diet significantly increased DHA level (p < 0.001), showing that this fatty acid is a good marker of fish consumption, even of fish species with a low fat content. A diet without fish incorporation for two months promoted a significant increase of AA (p < 0.001) and a decline of n-3 PUFA (p < 0.001).