Although the results of the National Wilms' Tumor Study suggest poor outcome for children with metastases to intra-abdominal, intracranial, skeletal, skin, diaphragm, or mediastinal sites, the incidence and course of mediastinal disease has not been described. Fifty-four newly diagnosed Wilms' tumor patients at Children's Hospital of Michigan (1972-1980) were studied for patterns of intrathoracic metastases. All had favorable histology; mean age (MA) at diagnosis was 4.5 years. Initial chemotherapy ranged from single agent to three-drug combination (actinomycin, vincristine, adriamycin). Intrathoracic metastases occurred in 14 (26%) from 0-28 months (median, 3.5 months) from diagnosis (MA, 5.8 years). Upon the discovery of intrathoracic metastases all received radiation (1,200-2,000 rads) to the chest, some with supplementary doses to the mediastinum. Five children with parenchymal and one with parenchymal and hilar metastases appearing 0-28 months after the time of diagnosis survive disease-free 44-127 months after metastases appeared (MA, 6.3 years). Eight children developed parenchymal and mediastinal metastases (MA, 5.4 years). Time from diagnosis to parenchymal metastases was 0-7 months; time from diagnosis to mediastinal metastases was 0-16 months; and in four, both sites appeared simultaneously. Death occurred in all eight patients 7-25 months from diagnosis. Only one child had evidence of recurrence outside the chest. Potential risk factors and influence on survival are discussed.