Japanese are unique among the peoples of developed count-ries in having a high intake of eicosapentaenoic acid (C 20:5) from fresh fish and this may in part contribute to their low incidence of cardiovas-cular diseases. Mass spectroscopic analyses of eicosapolyenoic acids (C 20:3, C 20:4 and C 20:5) were carried out on the serum of aged persons living on Kohama island in Okinawa and known to have the lowest incidence of cardiovascular diseases in Japan. All but 4 of the 77 persons examined (73.94±7.81 years old) led active fishing-farming lives. The total amount of eicosapolyenoic acids in the serum of persons on Kohama island (46.77±7.46 mg/ 100 ml) was higher (p<0.001) than that in people on mainland Japan, owing to the higher intake of fresh fish (147.7g/day). A positive correlation (p<0.01) was found between serum C 20: 5 concentration (6.82+2.54 mg/100 ml) and high density lipoprotein concentration (55.38±13.83 mg/100 ml). In addition, there were positive correlations (p<0.01) between serum C 20: 3 concentration (6.58+1.61 mg/100 ml) and total cholesterol (188.60±32.30 mg/ 100 ml), and triglyceride and skinfold thickness. The blood pressure level (p<0.01), incidence of abnormal ECG (p<0.05), and salt intake (6.2-8.3g/ day) estimated from urinalysis, were all lower than the average figures for Japanese of similar ages. No persons examined showed Q-wave on ECG. The percentage of smokers and drinkers were similar for Kohama island and mainland Japan.