Red blood cell aggregation in vitro (kinetics and shear resistance) was studied in 13 healthy controls and 13 type I (insulin-dependent) diabetic patients free of severe degenerative complications who were matched for age, sex, and body mass index. Measurements were performed with a device that analyzes the laser light backscattered by a blood suspension. Both the velocity of rouleau formation and the cohesion of the rouleau network were significantly increased in diabetic patients. Plasma viscosity and whole-blood viscosity measured at low shear rate (0.95 s-1) were also significantly elevated in the diabetic group. Multivariate analyses of the whole population sample and the diabetic patients confirmed the influence of plasma proteins on the kinetics of aggregation. Fibrinogen levels, which were close to normal, affected mainly the shear resistance of the aggregates. Triglyceride and apolipoprotein (apo) B levels and indexes of metabolic control or protein glycation (fasting blood glucose and fructosamine) also appeared to influence markedly both the kinetics of rouleau formation and the cohesion of the rouleau networks. These rheological abnormalities occurred in diabetic patients before the appearance of any severe degenerative complications. We suggest that these rheological abnormalities are linked to plasma or erythrocyte factors, and are not due to angiopathy.