Alzheimer's disease in 22 twin pairs--13-year follow-up: hormonal, infectious and traumatic factors.

We obtained follow-up data on 22 sets of twins where at least one twin had Alzheimer's disease (AD). The concordance rate for monozygotic twins (n = 17 pairs) was 59%, whereas that for dizygotic twins was 40%. In our series 8 monozygotic twins had hysterectomies; all had AD. The twins with hysterectomies also had a tendency to develop AD at an earlier age than their co-twin. Five twins with serious systemic infection developed AD, and they tended to have earlier onset than their corresponding twin. We found no strong evidence that head injury predisposed to AD.