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Introduction: Latent infection with Toxoplasma gondii has been associated with behavioral and cognitive changes in animal models and in humans. Early findings have suggested an association between latent toxoplasmosis and Alzheimer disease (AD). On the basis of these factors, we sought to determine whether there is an association between latent toxoplasmosis and AD using a large, well-characterized sample of subjects with AD and age-matched and sex-matched controls without dementia. Methods: Using ELISA, we determined anti-T. gondii IgG antibody titers in 114 control subjects and in 105 subjects diagnosed with AD through an Alzheimer's Disease Research Center. Results: There were no group differences between groups in age, ethnicity, or sex. Education and socioeconomic status was slightly higher in the control group. Neither the prevalence of anti-T. gondii IgG antibodies (33% in the nondemented control group compared with 41% in the AD group, P= 0.25) nor log-transformed antibody concentration (106.6 IU/mL in the control group compared with 140.9 IU/mL in the AD group, P= 0.85) differed between the control and AD groups. Discussion: In this sample, we found neither a higher prevalence of latent toxoplasmosis in the AD group compared with the control group nor differences in serum anti-T. gondii IgG titers between groups.