Laparoscopic Roux-en-Y gastric bypass is the most commonly performed surgical procedure for obesity and, consequently, post-operative patients are increasingly encountered by all specialties. This is a case of a patient presenting with abdominal pain, nausea and fever 9 months following gastric bypass surgery caused by diffuse large B-cell lymphoma (DLBCL) in the bypassed stomach. It demonstrates well that symptoms that may normally be considered ‘red-flags’ may not be as obvious or specific following an operation. The case also indicates the importance of considering diagnoses unrelated to surgery presenting in the post-operative period (especially when conventional investigation methods are not feasible), and the potential danger of assuming they are due to the operation alone; had this occurred in this patient then a malignancy may have been missed. This is only the second reported case of DLBCL in the bypassed stomach, and the third for lymphoma of any type.