Analysis of the plasma and erythrocyte fatty acid composition of poorly controlled, insulin-dependent (type I) diabetic patients revealed a low level of 18:3c, omega 6, 20:3c, omega 6, 22:6c, omega 3 and total polyunsaturated fatty acids, and a high level of 18:0 and total saturated fatty acids. Improvement of diabetic control, achieved by treatment with continuous subcutaneous insulin infusion coincided with an increase of arachidonic acid and a normalization of total polyunsaturated fatty acids, with a concomitant decrease of total saturated fatty acids and total monounsaturated fatty acids, especially 18:1c, omega 9. The results of this study can be important for the dietary advice to poorly controlled, diabetic patients.