Eight capuchins approximately 8 to 12 mth old were maintained on diets supposed to contain all the essential dietary elements, with only the intake of vitamin D subject to variation. Essential fatty acid (EFA) requirements were supplied by corn oil. All the monkeys developed skin lesions, and two were destroyed after self mutilation. EFA deficiency was suspected from the symptoms and organ histology. Analyses of red cell membranes and biopsy and autopsy samples of tissues suggested a specific linolenic acid deficiency. Complete recovery occurred when purified linseed oil was added to the diet.