In order to elucidate the tendency to erythrocyte aggregation in gestational diabetes, blood viscosity at a standard hematocrit of 45% (corrected viscosity) was measured at low shear rates in the third trimester of normal pregnancy and pregnancy with gestational diabetes. The mean values (+/- S.D.) for corrected viscosity of 20 gestational diabetic patients were 69.2 +/- 16.6cp at shear rate of 0.11 sec-1, 34.9 +/- 5.3cp at 0.54sec-1 and 27.9 +/- 5.0cp at 0.99sec-1. They were significantly higher than those of 40 normal pregnant patients (55.2 +/- 11.4cp at 0.11sec-1, 29.0 +/- 2.6cp at 0.54sec-1 and 24.4 +/- 2.4cp at 0.99sec-1). The mean value of fibrinogen of gestational diabetic patients was 408.8 +/- 106.7 mg/dl and significantly higher than that of normal patients (313.3 +/- 65.1 mg/dl). The increased tendency to erythrocyte aggregation observed in the diabetic patients was due to increased fibrinogen and would induce microcirculatory disturbance and mild hypoxia in the placental tissue. The cytotrophoblastic cell proliferation which is a characteristic morphological finding of diabetic placenta would be induced as a result of the reaction of placental tissue to the hypoxia thus produced.