OBJECTIVE: To determine whether there is an association between the number of peripheral T cells binding IgM per total T cell population (%IgM+ T cells) and dementia. DESIGN: Cross-sectional study. SETTING: Two inpatient and two outpatient sites at a university medical center. PARTICIPANTS: Fifty three adults. MEASUREMENTS: Peripheral blood was collected from each individual, and the %IgM+ T cells was determined by flow cytometry. The data obtained by medical record chart review were analyzed to determine whether the %IgM+ T cells correlated with cognitive diagnoses, demographic variables, medical diagnoses, or prescribed medications. RESULTS: The %IgM+ T cells was negatively correlated with MMSE scores (r = -.33, P = .016). There was a significant difference in the mean %IgM+ T cells between Alzheimer's disease (AD) and non-AD patients (35.6% ± 30.2% vs 14.6% ± 23.9%, P < .001) but no statistically significant association between the mean %IgM + T cells and age, sex, race, prescribed medications (except gastrointestinal (GI) medications), or medical diagnoses (except stroke). After statistically controlling for GI medications and stroke, AD remained independently associated with the %IgM+ T cells (P = .008). CONCLUSIONS: Patients with AD had significantly more of their T cells coated with IgM than did non-AD patients.