Treatment of severe obesity by Roux-en-Y gastric bypass (RYGB) causes sustained weight losses and improves health complica-tions. RYGB is, however, also associated with adverse effects. Dumping is a well-known complication causing invalidating symptoms, and lately there have been mounting concerns about post-RYGB hypoglycaemia. This condition is characterized by neuroglycopenia and inappropriately elevated insulin concentration. The mechanism behind this hypoglycaemia is not completely elucidated. Diagnostic procedures and treatment possibilities are discussed in this article.