Eight patients with type IV and V hyperlipoproteinemia were put on a mackerel and herring diet of an isocaloric regimen for 2 weeks, in a cross-over design. At the end of the dietary periods a predominant increase of eicosapentaenoic acid (EPA - C20:5, n-3) in cholesterol esters and of docosahexaenoic acid (DHA - C22:6, n-3) in serum triglycerides, being more pronounced after mackerel as compared to herring diet, could be confirmed. After mackerel diet serum triglycerides and total cholesterol were significantly lower, returning to basal levels 3 months later. High density lipoprotein (HDL) cholesterol appeared slightly increased after mackerel diet and decreased to initial values thereafter. After herring diet, which contained half as much EPA as compared to mackerel diet, the differences were minor. The decline of free fatty acids (FFA) and insulin at the end of the mackerel period reached the level of significance 60 min and 120 min, respectively, after glucose load. A significantly lower systolic blood pressure in recumbent and upright position after the mackerel period could be found, whereas diastolic pressure and blood pressure after herring diet remained unchanged.