A case-control study was performed to determine the possible roles of various environmental factors, prior illnesses, drug use, and personal habits in the development of Alzheimer's disease. Such information was collected from 40 patients with onset of dementia prior to age 70 and from 80 community control subjects matched for age, sex, and race. No significant differences were found between patients and control subjects in toxic environmental exposures, animal contacts, smoking, drinking, or unusual dietary habits. A significantly higher frequency of prior thyroid disease was found in women patients than in women control subjects (25.0% and 7.1%, respectively). A history of severe head injury was also obtained significantly more often among the patients than among the controls (15.0% and 3.8%, respectively). Aside from these differences, which may prove to be important associative factors in this illness, there appeared to be no major premorbid demographic or clinical factors associated with this form of dementia. There was evidence, however, of a genetic factor that was manifested in an excess of dementia and mental retardation (including Down's syndrome) in families of patients with Alzheimer's disease.