Mitral valve prolapse is more prevalent in patients with autoimmune diffuse toxic goiter, suggesting a possible etiologic association. The prevalence of mitral valve prolapse was determined in 75 patients with chronic lymphocytic thyroiditis, another autoimmune thyroid disorder, and in 50 healthy control subjects. Mitral valve prolapse was found in 31 of 75 (41%) patients with chronic lymphocytic thyroiditis and in 4 of 50 (8%) controls (odds ratio, 8.10; 95% confidence interval, 2.64 to 24.83; p less than 0.0005). The age-race adjusted odds ratio was 9.26 (95% confidence interval, 2.83 to 30.26; p less than 0.0005). No correlation between the prevalence of mitral valve prolapse and the presence of hypothyroidism, serum antithyroid antibodies, or the duration of chronic lymphocytic thyroiditis was found. The prevalence of mitral valve prolapse is substantially increased in patients with the autoimmune thyroid disorders.