In some multiple-case families, dementia of the Alzheimer type (DAT) is very likely to be of a genetic origin. Nevertheless, most DAT cases are sporadic and a disease with a majority of sporadic cases is usually of environmental origin. Are there two different etiologies of DAT? And if so, what is the proportion of genetic cases? We describe a model designed to assess the genetic risk in DAT. Our basic hypotheses were: (1) DAT could be of either genetic or environmental origin; (2) in its genetic form, transmission is of the autosomal dominant type, and (3) a proportion (pi) of the population is exposed to environmental risk. We have estimated the value of the probability (pi) of being exposed to the environmental risk and of the gene frequency (p). These values were highly dependent on the model assumptions, but non-inherited cases were consistently more frequent than inherited ones.