Hyperplasia of the thymus is a normal physiologic response in infants and children during recovery from life-threatening illness. New, recurrent, or residual mediastinal masses in children treated for malignant disease present a diagnostic dilemma. Are these masses recurrent disease or simply normal reactive thymic hyperplasia? Our experience from 1979 to 1986 includes 14 children aged 1 to 17 years (mean 7.4 years) who were identified with new or recurrent mediastinal masses during or after chemotherapeutic treatment for malignant disease (lymphoma 9, Wilms tumor 2, leukemia 1, osteosarcoma 1, malignant teratoma 1). The mediastinal masses were treated by a variety of methods depending on the attending physician's preference (close observation 2, oral steroids 5, steroids and subsequent biopsy 1, open biopsy 6). Chest roentgenograms of "observation only" patients have showed stable mediastinal changes without clinical evidence of recurrent disease. Patients treated with steroids showed resolution of the mediastinal masses in 48 hours to seven days, without recurrence. Patients undergoing open biopsy showed only thymic hyperplasia and/or lymph nodes. We suggest a stepwise approach to evaluation of these patients. Mediastinal masses occurring during, or shortly following, chemotherapeutic treatments of malignant disease should first be treated with oral prednisone (60 mg/m2/d x 7 to 10 d). If the patient shows a complete or partial resolution, then follow-up includes frequent chest roentgenograms and/or a second course of steroids. If the mass fails to respond to steroids, or enlarges, then open biopsy through a minithoractomy will clarify the diagnosis. Follow-up of our patients is from 3 months to 7 1/2 years (mean 5 years).(ABSTRACT TRUNCATED AT 250 WORDS)