In vitro measurements were carried out to study the aggregation of erythrocytes in 33 children and young adolescents, three older adolescents, and 38 adults with type I diabetes. The aggregate formation of erythrocytes in stasis was increased in adult patients with both "good" and "poor" metabolic control when compared to control values. The aggregation of red cells in all children and the younger adolescents, both those under good and poor metabolic control, did not statistically differ from those of controls. Each of the three older adolescents showed an increased aggregate formation of their erythrocytes compared to controls. All adult patients under poor metabolic control exhibited various stages of diabetic retinopathy as shown by fluorescence angiography. Only one child under poor metabolic control showed an early stage of retinopathy (stage I according to Malone). The three older adolescents showed an increased aggregate formation of their erythrocytes. These patients exhibited also stage I of retinopathy. We suggest that the increased aggregation of erythrocytes of both adult and older adolescent patients may be one of the reasons for the development of diabetic retinopathy. Moreover, the normal erythrocyte aggregation of the diabetic children and younger adolescents could help to explain the rare occurrence of microangiopathies in childhood diabetes.