We report the largest group of patients to date with hyperinsulinism (HI) and Beckwith-Wiedemann syndrome (BWS). Our study shows that BWS children with severe, persistent HI have a particular form of BWS caused by two copies of part of chromosome 11 inherited from the father (paternal uniparental isodisomy for chromosome 11; pUPD11p). Three-fourths of these children required intensive treatment for their HI, including pancreatic surgery in half of the cases. Pancreas samples from these children showed large areas of overgrowth of insulin producing cells. In most cases, there was no other identified genetic cause of HI, suggesting that pUPD11p alone can cause severe HI. Recognizing BWS in children with HI is important because children with BWS require long-term monitoring for embryonal tumors. (By Dr. Jennifer Kalish, http://jmg.bmj.com/content/early/2015/11/05/jmedgenet-2015-103394 )