Solid mediastinal masses in infancy and childhood occur most frequently in the posterior mediastinum. From 1972 to 1989, 63 patients presented with a posterior mediastinal mass. The median age at diagnosis was 6 years (range, 1 day to 26 years). Thirty patients were female. Forty-five percent of the patients presented with respiratory symptoms or chest pain; 13% had neurologic symptoms, one half of which were related to spinal cord compression; and 5% had a palpable mass. In 32% of patients the mass was an incidental finding. The tumors were of neurogenic origin in 89% of patients, of which neuroblastoma was the most common. Of all patients with posterior mediastinal masses, 60% had malignant tumors. Median follow-up for 62 of 63 evaluable patients was 45 months (range, 1 to 289 months). One patient was lost to follow-up. Of the 62 patients followed, 84% are alive and free of disease. All but 4 of the 32 patients with neuroblastoma are alive and free of disease with a median follow-up of 73 months (range, 7 to 289 months). Patients with neuroblastoma who were diagnosed in the first year of life had a significantly better survival pattern than those presenting after the first year. There were seven deaths in the series: four from neuroblastoma, two from primitive neuroectodermal tumor, and one from malignant schwannoma. Preoperative diagnostic evaluation of a posterior mediastinal mass should include posteroanterior and lateral chest roentgenograms, and either CT or MRI of the chest and abdomen to assess the extent of the mass.(ABSTRACT TRUNCATED AT 250 WORDS)