Eighteen computed tomography (CT) studies of the lungs prior to thoracotomy in 15 children with presumed pulmonary metastases were compared retrospectively with surgical and pathological findings. Computed tomography detected 86% of the malignant lesions and technical factors were thought to be important where larger nodules were missed. Although CT does not differentiate benign and malignant lesions, 90% of lesions detected at CT in this study contained tumour. In four patients it was found that nodules that increased in size on serial CT contained viable tumour, whereas nodules that decreased in size while a patient was on chemotherapy might or might not contain viable tumour.