This report describes 12 patients with the Wiedemann-Beckwith syndrome (WBS), including 6 familial cases from 2 families. The clinical manifestations do not allow for a differentiation between familial and sporadic cases. Consistent morphologic features include organomegaly, cytomegaly and nucleomegaly. The pathogenetic process may involve few or many organs and tissues and may represent a nuclear/mitotic dysfunction. Clinically, the manifestations are hyperplasia, hypoplasia, dysplasia, neoplasia and defects in differentiation. Secondary functional disturbances are at times prominent. The differential diagnosis of the WBS includes 1) the Wilm's tumor (WT)-aniridia syndrome: 2) the "tumor-hypertrophy syndrome" which includes WT, adenocortical tumors or hepatoblastoma; 3) the WT-pseudohermaphroditism syndrome; and 4) the "tumor-nevus syndrome" with or without malformations (particularly duplications) of the urinary tract. The latter two conditions are apparently not associated with hemihypertrophy. Familial occurrence suggests that some cases of the WBS may be due to delayed mutation. Carriers of the premutated allele appear to belong to two classes: those with a high risk of producing affected offspring and those who transmit the premutated allele but have no affected offspring.