Follow-up serial computed tomographic scans of 124 patients treated for testicular cancer with either radiotherapy or retroperitoneal lymphadenectomy alone or in combination with chemotherapy were evaluated. Thymic enlargement occurred three to 20 months after initiation of treatment in 15 of the 124 patients. Thymic enlargement could histopathologically be demonstrated in seven patients as true hyperplasia. One of these seven patients however had evidence of metastatic disease with thymic infiltration by a malignant teratoma. Thus it may be impossible to distinguish benign thymus hyperplasia from tumor-infiltration on the basis of ct information alone and sternotomy may be required. No severe defect of cellular immunity could be found. There is no specific constellation of lymphocytic markers in peripheral blood which could indicate true thymic hyperplasia.