Two tests for genetic heterogeneity within the framework of linkage analysis were performed among 18 caucasian families with Alzheimer's disease, for each of 27 phenotypic markers. Both tests were performed twice, first assuming that individuals with Down's syndrome were also "affected" with Alzheimer's disease, and second assuming that they were not. No statistically significant heterogeneity was found under either test, regardless of the affected status assumption. However, trends in the data were consistent with heterogeneous etiology between families with both Alzheimer's disease and Down's syndrome versus families with Alzheimer's disease only. The trends were stronger when the Down's syndrome members were considered affected, and were most striking for linkage with the MNSs locus.