An increased prevalence of mitral valve prolapse has been found in Graves’ disease and a common autoimmune etiology has been suggested for both disorders. We investigated the prevalence of mitral valve prolapse in 87 patients with autoimmune chronic lymphocytic thyroiditis, 50 patients with nongoitrous hypothyroidism and 111 healthy control subjects. Mitral valve prolapse was found in 16.09% of patients with chronic lymphocytic thyroiditis compared to 6 and 5.4% in nongoitrous hypothyroidism and normal controls, respectively. The result is statistically significant, p < 0.02, and confirms that the prevalence of mitral valve prolapse is significantly increased in patients with autoimmune disorders of the thyroid gland, when compared to normals and nonautoimmune conditions.