An excess of irregularly distorted red cells with spiked forms (acanthocytes, spur cells) has been found in a substantial minority of patients with senile dementia of Alzheimer type (7 of 50 patients, 3 of 21 men and 4 of 29 women). Of 100 control patients, 42 men and 58 women), 5 (3 men and 2 women) showed comparable distortion, but, of these, one man may well have incipient dementia and the others had serious organic diseases which may be associated with comparable erythrocytic changes. The cause of the distortion is not yet clear, but the presence of occasional giant erythrocytes in the absence of general macrocytosis suggests a possible abnormality of cell-membrane synthesis. This distortion may be a useful marker in patients with loss of memory. Whether it is a manifestation of a haemopoietic clone or a constitutional anomaly associated with Alzheimer's disease remains to be seen.