This study was designed to determine the frequency and clinical characteristics of myasthenia gravis (MG) in a large cohort of Persian patients with multiple sclerosis (MS) living in the province of Isfahan. We reviewed the case records of patients with definite MS (McDonald's criteria) registered in the Isfahan MS Society (IMSS) for associated MG. Of the 1,718 patients with MS in the registry, six patients were found to have both MS and MG. The prevalence after excluding one patient with insufficient data, was 291 per 100,000 (0.29%), a higher prevalence than the earlier reports. These results may support the hypothesis that MS and MG share some common immunopathogenic mechanisms.