Previous studies of red cell deformability (RCD) in diabetic patients have produced conflicting results. We have therefore reassessed this problem with the Carri-Med Filtrometer, which measures RCD independently of the rate of clogging of the filter. The initial red cell filtration ratio relative to buffer (mean +/- SD) of 69 diabetic patients (28 type 1 and 41 type 2) was significantly impaired in both type 1 (0.470 +/- 0.047) and type 2 diabetic patients (0.488 +/- 0.065), when compared to 66 non diabetic control subjects (0.540 +/- 0.032; both p less than 0.001). A significant difference in RCD was noted between type 1 and type 2 diabetics (p less than 0.03), but no association with vascular complications was found. No significant correlations were noted between RCD and the duration of diabetes, age, HbA1, leucocyte count, or mean red cell volume. However RCD was inversely related to mean red cell haemoglobin concentration (r = -0.43, p less than 0.01). Diabetics had significantly lower mean red cell volume and significantly higher mean cell haemoglobin concentration than non-diabetics, but these changes could not explain the difference in RCD, which may be related to alterations in the red cell membrane.