Relatives of probands with histologically confirmed Alzheimer's disease had excessive morbidity from Alzheimer's disease, Down's syndrome, and hematologic malignancies. These associations coupled with two previously reported ones, the indistinguishable histopathological changes in brain in Alzheimer's disease and Down's syndrome, and the 20-fold increased incidence of leukemia among persons with Down's syndrome, are evidence that some instances of those disorders are associated with a unitary genetic etiology. The genetic defect may be expressed through disorganization of microtubules. Other evidence suggests that the same process may be involved in aging and in other chromosomal aberrations.