A 4-year-old girl developed right metachronous Wilms tumor 2 years after completing treatment for a left-sided stage I Wilms tumor. The original treatment included 7 weeks of chemotherapy, delayed nephrectomy, and another 3 weeks of chemotherapy. The metachronous tumor on the right side extended into the inferior vena cava and right atrium. She developed pulmonary embolism as a result. She received chemotherapy and developed liquifaction of the tumor and toxic shock. She also had surgery. The patient is alive 3 years after the original diagnosis and 10 months after the relapse. The authors report this unusual case and discuss whether these cases can be identified early.