The following unsaturated fatty acids were examined in the serum and in the high density lipoproteins of 38 patients with diabetes mellitus type Il and 20 healthy controls: linolic, linolenic, eicosandienoic, eicosanetrienoic, arachidonic and eicosanepentaenoic. In decompensates diabetes mellitus a decrease of linolenic, eicosanedienoic, arachidonic and eicosanepentaenoic acids and an increase of linolic and eicosanetrienoic acids were found. After compensation of carbohydrate metabolism the last fatty acids showed a tendency toward normalization. The parallel changes of the fatty acids studied in the serum and in the high density lipoproteins suggest that the fatty acids contents of the HDL depends on the serum fatty acids. It is most probable that the variable changes of the individual unsaturated fatty acids play a role in the complex mechanism of vascular lesions in diabetes mellitus.