Fifty-three patients were investigated by Se methionine scan in order to evaluate the role of the thymus gland in myasthenia gravis. The thymus of 28 of the above patients was examined histologically. Five out of six thymomas were detected with this technique and eight out of twelve hyperplasias were revealed. The above data suggests that Se methionine scan is a useful, innocuous tool for diagnosing neoplasms and abnormal germinal centers in thymus glands.