A clinical case of linoleic acid deficiency has been investigated because of the unknown effect in an adult man. The patient had had all but 60 cm of his small intestine removed and was maintained solely by intravenous therapy for 100 days, without fat. At this stage there was no clinical evidence of malnutrition, but his serum phospholipids were found to contain 10% 5,8,11 eicosatrienoic acid and he developed a skin rash. A soybean fat emulsion containing 86 g/l of linoleic acid was given intravenously for 12 days, discontinued for 43 days and then resumed. During each period of fat administration (linoleic acid intake 22.8 g/day) the serum phospholipid content of eicosatrienoic acid fell and arachidonic acid rose. Simultaneously the serum triglycerides fell to normal, the proportions of the triglycerides carried by the lipoproteins (d < 1.006) rose to normal and the rash disappeared. A second case of linoleic acid deficiency, in this instance associated with severe malnutrition, was investigated and here also a high level of plasma triglycerides was mostly associated with lipoproteins with d < 1.006. In an attempt to estimate his minimal daily requirements, feeding 7.5 g/day of linoleic acid in this form did not eliminate the eicosatrienoic acid, but instead allowed a small increase. It is concluded that adult man requires at least 7.5 g/day of linoleic acid and that the proportion of the triglycerides carried by the very low density lipoproteins is lowered in the absence of linoleic acid.