A firm relationship has been established between thymus gland pathology and myasthenia gravis. The most frequent changes observed in this gland consist in simple hyperplasia of germinal centers or thymoma. In order to detect these alterations it is necessary to have a suitable reliable technique, beside routine radiological exams: thymus scan has been proposed as an useful tool. Since 1968 we submitted 52 patients to this procedure; we used as a tracer 75Se-Seleniomethionine in most cases, 57Co-bleomycin and 67Ga-citrate in a few cases. The scan was found positive in 22 cases (42%) showing either localized or diffuse uptake in the region of the sternum. Thymus changes were subsequently examined in 26 cases: 22 cases after thymectomy in 4 cases at autopsy. We found a good correlation between positive scan and pathological changes of the gland such as thymoma or germinal center hyperplasia. However in 4 patients with negative scan that previously received radiation therapy or immunosuppressive therapy such pathology was missed: 3 cases of hyperplasia and 1 residual thymoma. We conclude that thymic scan an useful and highly specific diagnostic procedure and should be done before any kind of treatment which affects thymus.