The influence of fish oils rich in n-3 fatty acids on plasma lipid and lipoprotein concentrations in patients with primary hyperlipoproteinaemia is reviewed. When used as a dietary supplement, n-3 fatty acids exert their greatest effect on the concentrations of triglyceride-rich lipoproteins and these effects are dose dependent. Low doses of n-3 fatty acids reduce plasma triglyceride concentrations in patients with phenotypic type IV, type V and type IIB hyperlipoproteinaemia, but concurrently result in modest increases in the plasma concentrations of low density lipoprotein (LDL). With very high doses plasma concentrations of LDL cholesterol decrease in patients with the type IIB phenotype. At low doses n-3 fatty acids do not significantly lower concentrations of LDL cholesterol in patients with primary hypercholesterolaemia whereas at higher doses a modest LDL lowering effect is achieved. The results indicate that the therapeutic potential of n-3 fatty acids as hypolipidaemic agents is greatest in patients with hypertriglyceridaemia.