An instrumentation for carrying out a gastrointestinal bypass comprises means for bringing close together a first tissue portion (12) of the intestine and second tissue portion (14) of the stomach by realising a first intestine loop (A), means for creating a gastroenteroanastomosis (16) between the two close tissue portions, for example a linear stapler, maintaining or restoring the continuity of the intestinal duct after the realisation of the gastroenteroanastomosis, means for bringing close together an additional first tissue portion (18) of the intestine, arranged distally with respect to the gastroenteroanastomosis (16) with reference to the natural flow of the food along the intestinal duct, and an additional second tissue portion (20) of the intestine arranged proximally with respect to the gastroenteroanastomosis (16) with reference to the natural flow of the food along the intestinal duct, realising a second loop (B) of the intestine, wherein said additional second tissue portion (20) of the intestine arranged proximally with respect to the gastroenteroanastomosis (16) is brought close to the gastroenteroanastomosis (16), means for creating an enteroenteroanastomosis (22) between the two close tissue portions of the intestine, maintaining or restoring the continuity of the intestinal duct after the realisation of the enteroenteroanastomosis, and means for sectioning the intestine between the gastroenteroanastomosis (16) and the enteroenteroanastomosis (22).