Recently, it has been pointed out that platelets play a role in thrombosis and atherosclerosis, and attention has also been focused on the relation between platelets and polyvalent unsaturated fatty acids. Therefore, arachidonic acid (AA), eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA) and other fatty acids in the plasma of cases of thrombotic disease (TD) were analyzed. The conclusions were as follows: (1) The EPA/AA ratio in the TD group was 0.40±0.22 (M±SD) which was significantly lower than that of the control group (0.87±0.48) (P≤0.01). (2) The DHA/AA ratio in the TD group was 0.77±0.20, less than the 1.12±0.40 value of the control group. From these results, it is assumed that the low values of the EPA/AA and DHA/AA ratios in the plasma are risk factors in the onset or worsening of thrombotic disease.