The volumes of two neuroblastomas and two Wilms' tumor masses were calculated by computerized tomography (CT) within 48 hours of surgical removal of the tumor. True volumes of the tumor masses were obtained by volume displacement. CT was very accurate in predicting volume with an error ranging from 1.5% to 7.5%. This accurate method of evaluating tumor volume will allow precise monitoring of the response of nonoperable tumors to chemotherapy and radiotherapy.