The number of obese people with diabetes mellitus (DM) is increasing. DM is in itself a risk factor for all types of infections, including infections after total hip and knee prosthetic surgery. In particular, the prevalence of obesity is high among orthopaedic patients. Obesity is also a risk factor for post-operative infections in general. After total hip arthroplasty obese patients have a threefold increased risk for post-operative infections compared to a patient with normal weight. There is an association between DM and obesity, and this association is explained by the presence of metabolic syndrome. Fat tissue in people with abdominal obesity acts as an endocrine organ and metabolic syndrome is characterized by a condition of chronic inflammation, insulin resistance, and vascular obstructive disease. The obese patient with DM should be considered a high risk patient for post-operative infection in all types of surgery. Efforts should be made to identify patients with a metabolic syndrome prior to prosthetic surgery. Fasting glucose levels and HbA1C should be included in pre-operative blood tests, and, if necessary, specialists in internal medicine and endocrinology should assess the patient pre-operatively. The amount of antibiotics administered as prophylaxis should be adjusted to the body mass and the blood glucose level should be controlled per-operatively. Persistent wound drainage for more than five days post-operatively should be considered for surgical revision. If possible, the patient should of course lose weight before the operation, but the role of bariatric surgery is not discussed in this presentation.