From 1970 to 1988, 121 patients younger than 18 years of age who had newly diagnosed Hodgkin's disease were treated at the Children's Hospital of Philadelphia (CHOP) and the Hospital of the University of Pennsylvania (HUP), Philadelphia, Pennsylvania. Fifty-five of 79 children with mediastinal masses (MM) had pretreatment chest radiographs from which a mediastinal mass ratio (MMR) could be calculated. Within a range of MMR values, 0.25 was the best prognosticator for event-free survival (EFS) for all patients. In those treated with radiation therapy (RT) alone, the intrathoracic relapse rate was zero of five patients with small MM (MMR less than 0.25) versus five of eight patients with large MM (P = 0.09). For combined-modality therapy (CMT), there were intrathoracic relapses in zero of four patients with small MM versus 5 of 32 patients with large MM (P = 0.8). For CMT, the intrathoracic relapse rates for those receiving more than 3500 cGy versus less than 2500 cGy were 0 of 4 patients and 5 of 27 patients, respectively (P = 0.8). The intrathoracic relapse rate in children with large MM was significantly lower for CMT than for RT (5 of 32 patients versus 5 of 8 patients) (P = 0.02). The authors concluded that in pediatric Hodgkin's disease, a MM with a MMR greater than or equal to 0.25 may be associated with poor intrathoracic control after RT alone. Despite this, children with large MM treated with RT alone had an excellent overall survival rate.