1. 1. Rats placed on a diet free of certain unsaturated fatty acids develop in the early stages scaliness of the skin and in the later stages necrosis of the tail. Returning the essential acids to the food leads to immediate improvement in the skin condition. 2. 2. Infants with eczema may be given plenty of fats (corn or linseed oil) rich in unsaturated fatty acids, and some of these children show improvement. 3. 3. Examination of the plasma lipids of the rats with scaliness of the skin and the infants with eczema reveals that the unsaturated fatty acids are below the average level in the majority of the cases. After the ingestion of the acids, the serum unsaturated fatty acids usually increase to within the normal range. 4. 4. The conclusion has been reached that the low unsaturation of the serum fatty acids in infantile eczema is one of the important causes of the disease. 5. 5. In order to find out the result of a further reduction in unsaturated fatty acids, such as occurs in acute respiratory infections in infants, we subjected nine infants with eczema to periods of careful observation in the hospital. During these periods all infants had an attack of an acute disease. The skin changes were watched closely, and periodic determinations of the plasma lipids were made. 6. 6. The acute infections were accompanied by a short period of improvement in the skin condition at the onset of the disease, but this was followed by a rather rapid aggravation in the eczema. The height of the infection usually marked the peak of the flare-up in the condition of the skin, and this was also the time when the degree of unsaturation of the serum fatty acids was lowest. 7. 7. The assumption, therefore, is that the acute respiratory diseases cause the eczema to become worse through the medium of depressing the serum unsaturated fatty acids to low levels, and that improvement in the skin condition takes place during the convalescent period because there is a corresponding gradual return of the unsaturation of the fatty acids. 8. 8. In some instances the eczema responded much better to therapy after full recovery from the acute illness than before its onset. 9. 9. This has brought up the question of the influence of other factors, such as the fever, on the skin condition and infers that more studies must be carried out before definite conclusions can be drawn.