Synchronous primary malignancies are rare in paediatric patients and usually develop based on inherited cancer predisposition. We report the case of a 4-year-old girl with lateralised overgrowth who was diagnosed with concomitant B-cell precursor acute lymphoblastic leukaemia and Wilms tumour. Methylation analysis at chromosome 11p15.5 revealed a mosaic gain of methylation at IC1 on the maternal allele in kidney tissue, which was absent in leukemic and remission blood cells, confirming the diagnosis of Beckwith-Wiedemann syndrome. We discuss the cause-and-effect relationship for both malignancies and point out diagnostic and therapeutic difficulties that were encountered in this patient.