Gastric bypass (GBP) results in rapid type 2 diabetes (T2D) remission in most cases. Consequences of GBP reversal are unknown. A GBP-operated T2D patient was given mixed-meal tests before (MMTpre), 2 months (MMT2-M) and 12 months (MMT12-M) after GBP reversal. Glucose, hormones and metabolite profiles were assessed. MMT2-M displayed slightly lower glucose levels; MMT12-M displayed higher glucose and insulin levels, indicating deteriorating glycaemia. Homeostasis model assessment (HOMA)-β was higher at MMT2-M, but reduced at MMT12-M. Matsuda index revealed slightly reduced insulin sensitivity at MMT2-M, which deteriorated further at MMT12-M. Markers for metabolic stress and insulin resistance were elevated at MMT12-M. Gastric inhibitory polypeptide (GIP) levels were increased at MMT2-M and decreased at MMT12-M. Glucagon-like peptide-1 (GLP-1) decreased at MMT2-M and further decreased at MMT12-M. In conclusion, in this patient, GBP reversal provoked deteriorating glycaemia and long-term development of insulin resistance.