The autologous mixed lymphocyte reaction (AMLR) was used to study T lymphocytes in a group of patients with dementia of the Alzheimer type (DAT) in order to confirm the observation that their T cell proliferation in AMLR was greater than in age-matched controls, and to investigate other pathways of T cell activation, searching for correlations between immunologic and clinical findings in DAT. The mean proliferative response in AMLR was increased in patients with DAT. No differences between patients and age-matched controls were detected when other pathways of T cell activation were investigated. The degree of response in the AMLR varied among patients with DAT. This fits with the suggestion that the disorder may be a heterogeneous syndrome.