Blood rheology is now receiving increasing attention as an important potential contributory factor to diabetic angiopathy. This study was designed to provide evidence for and against early hemorheological abnormalities in diabetes mellitus (DM). For this purpose, blood viscosity, RBC aggregation, hematocrit, and plasma protein's levels of both fibrinogen and albumin were measured in 86 uncomplicated patients with DM (45 type 1 and 41 type 2). Patients with HbA1c < 7.5% were considered as having good glycemic control (GGC), while those with HbA1c > 8.5% as having poor glycemic control (PGC). Patients with type 1 DM showed normal blood viscosity at both shear rates high and low, while native hematocrit, fibrinogen, and RBC aggregation were significantly elevated and albumin significantly reduced when compared with healthy volunteers. Patients with type 2 DM showed more marked impairments associated with an increased low shear rate blood viscosity, when compared with patients with type 1 DM. Comparison between two subgroups of patients, both of which with type 1 DM and of similar disease duration of <5 yrs, with GGC or PGC showed that impaired blood rheology does depend on the quality of glycemic control. Differences were attenuated after a disease duration of >15 yrs. These findings suggest that early hemorheological impairments in patients with type 1 DM are dependent upon the glycemic control. In contrast, hemorheological impairments appear to be inevitable after a mean disease duration of 15 yrs even if there is a GGC. Aggravation of hemorheological abnormalities in patients with type 2 DM might depend upon the hemorheological effects of other metabolic abnormalities related to insulin resistance rather than the quality of glycemic control.