1. In a cross-sectional study, 32 insulin-treated diabetic patients had elevated low shear (27·1 vs 22·1 mPa s, P < 0·05) and high shear blood viscosity (5·4 vs 5·0 mPa s, P < 0·05) when compared with 10 non-diabetic controls. After correction to 45% packed cell volume, the abnormality had a tendency to be greater in patients with proliferative (mean low shear viscosity, 30·8 mPa s) than background (29·2 mPa s) or nil/minimal retinopathy (27·6 mPa s, 0·05 < P < 0·07, permutational trend test).
2. The fibrinogen levels were higher in the diabetic group (P < 0·05) and correlated with the low shear blood viscosity (rs = 0·38, P < 0·05).
3. In a prospective study, 74 insulin-treated diabetic patients with background retinopathy were randomized into two groups. Thirty-six patients were on attempted improved therapy (A group); in these the mean glycosylated haemoglobin (Hb A1c) fell within 1 year (11·6 to 10·1%, P < 0·001). Both the corrected low shear blood and plasma viscosity fell similarly (P < 0·001). The fall was greater than in those patients who were kept on usual therapy (U group) and whose glycosylated haemoglobin did not change significantly (11·7 to 11·4%) over the year.
4. The effect of diabetes on blood viscosity may not be a direct pathological factor, as the same increased viscosity would be produced by a mean increase of 1·7% in packed cell volume, compared with a population range of 14% packed cell volume.