To determine the incidence and relative risk (RR) of cancer in children with Beckwith-Wiedemann syndrome (BWS), children with BWS were followed up from birth until death, diagnosis of cancer, fourth birthday, or last day of follow-up. A total of 183 children with BWS were followed up for 482 person-years. The end points were incidence of cancer, RR of cancer, and RR associated with specific BWS phenotypic features. Thirteen children were identified with cancers before the fourth year of life in comparison with fewer than one cancer expected in this group on the basis of general population rates over the same period. The average annual incidence of cancer in the first 4 years of life was 0.027 cancer per person-year. The RR of Wilms tumor (RR = 816; 95% confidence interval [CI], 359-1156), neuroblastoma (RR = 197; 95% CI, 22-711), and hepatoblastoma (RR = 2280; 95% CI, 928-11,656) were statistically significant. Asymmetry of the limbs (hemihypertrophy) was the only clinical feature associated with an increased RR of cancer (RR = 4.6; 95% CI, 1.5-14.2). Given the high incidence of cancer in infancy and early childhood of patients with BWS, a prospective study is warranted to address the utility of screening for cancer.