Plasma and red cell fatty acids were measured in 44 children with third degree malnutrition: 22 children with marasmus, 11 children with kwashiorkor, and 11 with marasmic kwashiorkor (MK). The presence of edema, dermatosis, hair changes, and hypoalbuminemia were positively correlated with the red cell percentage linoleic acid and negatively correlated with red cell percentage arachidonic acid. Red cell percentage linoleic acid in kwashiorkor was decreased from the control value of 15.3% to 10.9%. In marasmus and MK, there were significantly greater decreases, to 7.7% and 8.0%, respectively. Red cell arachidonic acid in kwashiorkor was significantly (P<0.001) decreased to 9.8% (control value of 13.1%) whereas in marasmus there was no significant change. Only one sample, plasma from an infant with marasmus, had any detectable eicosatrienoic acid (20:3,ω 9). Similar changes in fatty acid composition have been reported in infants with acrodermatitis enteropathica, in an infant with biotin deficiency, and in biotin-deficient rats.