A lower erythrocyte deformability, which causes impairment of the microcirculation, is postulated to contribute to diabetic organ complications. Erythrocyte deformability was measured in four groups of type 1 (insulin-dependent) diabetic subjects and 30 controls by filtration and ektacytometry. Twenty-five patients without organ complications, 21 with microalbuminuria, 13 with overt nephropathy and 12 with leg ulceration were studied. No decreased erythrocyte deformability was found in any of the diabetic groups with either technique, and neither did the total group of 71 diabetic subjects have a lower erythrocyte deformability when compared with the controls. In order to imitate local conditions in the kidney, erythrocyte deformability was also measured in hyperosmolar solutions. Again no differences were found between the diabetic groups separately or as a whole and the controls. Furthermore no correlation was found between erythrocyte deformability and the plasma glucose or glycosylated haemoglobin level.