IN order to study the influence of the fat content of food on the occurrence of thrombosis in the arterial and venous system, a group of 133 hospital patients aged 65–90 years were treated with a diet in which butterfat, margarine and lard were replaced by vegetable oils (unhydrogenated corn oil and soybean oil). The daily intake of vegetable oils was about 40 gm., or half the total fats. The tune of treatment of the 133 patients ranged from 1 to 17 months (average 4.7 months) representing a total of 52½ years. For comparison, a control group of the same size and composition who received an ordinary hospital diet containing about 80 gm. animal fats (including about 40 gm, butter-fat, margarine and lard) was used. The occurrence of thrombosis and embolism in both groups was diagnosed clinically and post-mortem.