A 4-year-old child has been followed up in an oncology clinic for 18 months for chemotherapy and radiation therapy for stage III Wilms' tumor. As part of her routine clinic visit, a chest roentgenogram is obtained, and a new finding, a nodule, is observed in the right upper lung field. The pediatrician/oncologist has to decide whether the lung lesion should be considered to be metastatic Wilms' tumor or something else. Soon, there may be a new option, ie, nuclear magnetic resonance, which has the potential for distinguishing between benign and malignant tissue. If the lesion is benign, probably a histoplasmoma, nothing needs to be done. On the other hand, if the lesion is malignant, there are profound implications in terms of treatment plans.