Hemihyperplasia is characterized by asymmetric growth of cranium, face, trunk, limbs, and/or digits, with or without visceral involvement. It may be an isolated finding in an otherwise normal individual, or it may occur in several syndromes. Although isolated hemihyperplasia (IHH) is of unknown cause, it may represent one end of the clinical spectrum of the Wiedemann-Beckwith syndrome (WBS). Uniparental paternal disomy of 11p15.5 or altered expression of insulin-like growth factor 2 (IGF2) from the normally silent maternal allele have been implicated as causes of some cases of WBS. IHH and other mild manifestations of WBS may represent patchy overexpression of the IGF2 gene following defective imprinting in a mosaic fashion. The natural history of IHH varies markedly. An association among many overgrowth syndromes and a predisposition to neoplasia is well recognized. Heretofore the risk for tumor development in children with IHH was unknown. We report on the results of a prospective multicenter clinical study of the incidence and nature of neoplasia in children evaluated because of IHH. One hundred sixty-eight patients were ascertained. A total of 10 tumors developed in nine patients, for an overall incidence of 5.9%. Tumors were of embryonal origin (similar to those noted in other overgrowth disorders), including Wilms tumor, hepatoblastoma, adrenal cell carcinoma, and leiomyosarcoma of the small bowel in one case. These data support a tumor surveillance protocol for children with IHH similar to that performed in other syndromes associated with overgrowth.