Tumor and tumor-like lesions of the thymus, excluding Hodgkin and non-Hodgkin lymphomas, are among the least frequent mediastinal tumors in childhood. For this reason the diagnosis is often overlooked. The clinical and pathologic features of four thymus related masses, three of which were thymomas and one a thymic cyst, are reported in children between the ages of five and 15 years. The thymic cyst occurred in a child with a five year history of aplastic anemia. A rapid clinical course to death occurred in all four cases. The behavior of these lesions is compared with that described in the literature. The impression exists that thymomas in the pediatric age group are much more aggressive than those in adults. There are also some significant differences in their histopathologic appearance.