A recurrent mediastinal mass in a patient following treatment for Hodgkin's lymphoma presents a diagnostic challenge. We report a 17-year-old boy with nodular sclerosing Hodgkin's lymphoma, stage IIIsA, who achieved complete response after 6-cycle chemotherapy with doxorubicin, bleomycin, vinblastine and dacarbazine and developed thymic hyperplasia 6 months later. Ten cases have been reported previously. Most patients were young, had the nodular sclerosis subtype of Hodgkin's lymphoma, were in an advanced stage, and had a latency of 2 to 12 months following successful chemotherapy. It is impossible to delineate radiologically benign from malignant lesions. Resection of the tumor for tissue diagnosis is mandatory to establish an accurate diagnosis and thus avoid further harmful therapy.