Blood rheology (blood viscosity, haematocrit, plasma viscosity, erythrocyte sedimentation and deformability) was measured in 22 diabetics in a study to compare animal insulins with biosynthetic human insulin. All rheological variables were significantly abnormal in the diabetics when compared with matched normal controls, and were not influenced by the type of insulin injected. However, blood rheology was related to diabetic control, as judged by glycosylated haemoglobin levels; when diabetic control was worst (mean glycosylated haemoglobin 11%), blood viscosity was significantly higher and red cell deformability significantly lower than when control was at its best (mean glycosylated haemoglobin 9.5%). A similar correlation between mean red cell deformability and mean glycosylated haemoglobin was found when individual diabetics were compared (r = 0.66; p less than 0.001). The occurrence of abnormal blood viscosity and red cell deformability during periods of poor diabetic control may contribute to the development of diabetic vascular complications.