The presence of cancer and a congenital anomaly in the same child may be explained in certain cases by an underlying genetic abnormality. The study of these associations may lead to the identification of genes that are important in both processes. We have examined the records of 20,304 children with cancer in Britain, who were entered into the National Registry of Childhood Tumors (NRCT) during 1971-86, for the presence of congenital anomalies. The frequency of anomalies was much higher among children with solid tumors (4.4%) than among those with leukemia or lymphoma (2.6%; P < .0001). The types of cancer with the highest rates of anomalies were Wilms tumor (8.1%), Ewing sarcoma (5.8%), hepatoblastoma (6.4%), and gonadal and germ-cell tumors (6.4%). Cases of spina bifida and abnormalities of the eye, ribs, and spine were more common in children with cancer than among population-based controls. Future studies may be directed toward identifying the developmental pathways and the relevant genes that are involved in the overlap between pediatric cancer and malformation.