Sonography was used to evaluate 28 patients (age range, newborn to 16 years) with mediastinal and juxtacardiac masses and disease processes, partial or complete opacification of a hemithorax, and various juxtadiaphragmatic abnormalities. Technically adequate studies were obtained in 93% of the cases. Sonography was of greatest value in determining the cause of an opaque hemithorax, characterizing fluid collections, localizing fluid collections prior to percutaneous aspiration or drainage, and demonstrating the position and integrity of the diaphragm. Sonography was least successful in depicting and characterizing mediastinal disorders. The authors' method is described and the technical limitations and pitfalls of pediatric chest sonography are discussed.