A viscometric study of blood from insulin-treated diabetic patients is carried out. Patients are divided into three major groups—group I: without or with minimal retinopathy and recent diabetes (n = 37), group II: without or with minimal retinopathy and at least 20 years of diabetes duration (n = 35), group III: with severe retinopathy (n = 27). Each group is also subdivided according to the glycosylated hemoglobin (HbA_1c) level, used to assess long-term glycemic control. Finally, the rheological parameters of six groups are compared: three of which have a HbA_1c level less than 7.5% [I₁ (n = 15), II₁ (n = 9), III₁ (n = 9)] and three others have a HbA_1c level more than 7.5% [I₂ (n = 22), II₂ (n = 26), III₂ (n = 18)]. The most important result concerns the thixotropy index ξ_t, which reflects the dynamic property of red blood cell (RBC) disaggregability under shear. Strong correlations between this parameter and HbA_1c level are found for groups I (r = −0.53, p < 0.001) and II (r = −0.68, p < 0.001), providing evidence of a RBC disaggregability disorder for a poor glycemic control of diabetes. In contrast, such a correlation is not pointed out for the group II. As the value for ξ_t is not statistically different for groups II₁ and II₂ and is close to the normal value in both groups, the existence of a rheological protection against the retinopathy could be involved. If such a hypothesis was verified, the systematic measurement of the thixotropy index for insulin-dependent diabetic patients would allow the patients susceptible to retinopathy to be detected and, therefore, the insulin therapy to be optimized.