Appropriate dietary instruction on fat intake is required for Japanese to prevent coronary heart disease because there has been a consistent increase in fat intake during the past two decades. To gain fundamental information for dietary instruction, we examined the relation between dietary habits related to fat and serum fatty acid compositions in women aged 40-69 of four Japanese populations (fishing, coastal farming, inland farming and urban populations) and a Caucasian population in which different dietary habits have been reported. Among Japanese, dietary intake of monounsaturated and polyunsaturated fatty acids (in particular omega 6-polyunsaturated fatty acid) were higher in the urban population than in the other populations. Intake of omega 3-polyunsaturated fatty acids did not vary among the Japanese populations, but intakes of eicosapentaenoic acid and docosahexaenoic acid were higher in fishing and coastal farming populations than in the other two populations. Serum concentrations and compositions of omega 6-polyunsaturated fatty acids (mostly linoleic acid) were higher in the urban and inland farming populations than in the other two populations whereas those of omega 3-polyunsaturated fatty acids (in particular eicosapentaenoic acid) showed the opposite trend. The proportion of omega 6-polyunsaturated fatty acids was higher and that of omega 3-polyunsaturated fatty acids was lower in Caucasian women than in the Japanese populations. The ratio of omega 3 (polyunsaturated fatty acids) to omega 6 was highest in the fishing population (ratio = 0.38), intermediate in coastal farming (0.35), urban (0.27) and inland farming (0.24) populations, and lowest in Caucasians (0.07). Within populations, the percentage of serum omega 6-polyunsaturated fatty acids increased with the increase of frequency of meat intake and oil intake. The percentage of serum omega 3-polyunsaturated fatty acids increased with the increase in frequency of fish intake. Because there are evident associations between the frequency of fish, meat and oil intake and serum fatty acid compositions, a frequency questionnaire on these foods would be useful for dietary instruction in the prevention of coronary heart disease.