To determine whether there are clinical and epidemiological differences between familial and non-familial Alzheimer's disease, we studied 151 patients with the clinical diagnosis of Dementia of the Alzheimer Type (DAT). Eighty-four index patients (56%) had at least one relative with reported DAT-type memory loss; the remaining 67 did not. For comparison, patients with a positive family history were grouped into a suspected familial DAT (FDAT), and those with a negative family history were grouped into a suspected non-familial DAT (NFDAT). There were no significant differences between FDAT and NFDAT patients with respect to age of symptom onset, survival time, rate of cognitive decline, motor function, behavioral signs or symptoms, head trauma, thyroid dysfunction, maternal age, or birth order. There were no significant differences between pedigrees of FDAT and NFDAT patients in respect to the prevalence of Down's syndrome or age of mortality. To examine a more "familial" set of FDAT patients, an FDAT subgroup (F2DAT) consisting of 33 index patients with two or more affected relatives with DAT-type memory loss was compared to the 67 NFDAT patients in a similar analysis. No significant differences were found. However, NFDAT index patients had a higher reported prevalence of head trauma than those patients in the F2DAT subgroup (NFDAT 22%, F2DAT 7%, OR = 4.2, CI = 0.9-20.0, P = 0.08). Thus severe head trauma may be a more important etiological factor in the non-familial form of DAT. To better examine this tendency, future studies should be conducted utilizing a larger number of pedigrees and familial criteria which account for the number of persons at risk and the age at onset of disease.