1. Erythrocytes with membrane abnormalities seen on interference contrast microscopy (‘pitted erythrocytes’) were counted in venous blood samples from patients with treated coeliac disease and dermatitis herpetiformis, and from control subjects and patients who had had an elective splenectomy. 2. The percentage of ‘pitted erythrocytes’ was compared with the 99m Tc-labelled heat-damaged erythrocyte clearance, and with the splenic size computed from scintiscans. 3. There was a significant correlation between percentage ‘pitted erythrocytes’ and the above methods of measuring splenic function and size. Increase in the percentage of ‘pitted erythrocytes’ above the control range indicated splenic hypofunction. 4. The number of coeliac patients with percentage ‘pitted erythrocytes’ above the control range increased with increasing age at which they started a gluten-free diet. 5. Counting of ‘pitted erythrocytes’ is a simple and sensitive method of assessing splenic hypofunction in treated coeliac disease. It avoids radiation exposure, and is applicable to all ages and repeatable without risk. 6. The incidence of splenic hypofunction in coeliac patients may be related to the duration of untreated disease.