A space-occupying mediastinal mass was discovered in a 16-year-old boy with metastasizing non-seminomatous testicular tumour six months after successful conclusion of cytostatic chemotherapy. Metastases at other sites were excluded. An enlarged thymus was the only finding at operation. Histological and histochemical examination demonstrated diffuse hyperplasia. The findings demonstrate that reactive benign thymic hyperplasia after cytostatic chemotherapy has to be considered in the differential diagnosis of space-occupying mediastinal lesions. They can also occur in adults. They are benign, explainable as immunological rebound phenomena, and do not require operative intervention. In case of doubt, short-term corticoid administration will diminish the size of the mass and thus confirms the diagnosis.