To determine whether an association exists between Alzheimer's disease (AD) and thyroid disease, we carried out two studies in the population of Rochester, Minnesota. We reviewed medical records of a cohort of 198 women with histologically confirmed Hashimoto's thyroiditis (1935 to 1974) for evidence of subsequent dementia, applying the criteria used for dementia in a previous determination of incidence and prevalence rates in this population. From a total of 4,197 person-years of follow-up, eight cases of AD were diagnosed, whereas the expected number was 5.8. The standardized morbidity ratio was 1.37, which failed to reach statistical significance. The second study was a retrospective case-control comparison that sought any relationship between AD and all thyroid disorders, using a previously identified (1960 to 1979) AD cohort (N = 646) and their age- and sex-matched controls. For myxedema there was a positive association for AD without significance, whereas in Graves' disease there was a significant negative association for AD.