Limited available data for dosing in obesity of the medicines used in this case are discussed, with the emphasis on ertapenem. The case illustrates the difficulties in dosing medicines to morbidly overweight patients. The number of such patients is increasing but data on adequate doses of medicines are scarce. We demonstrate that ertapenem 1,5 g i.v. once daily provided adequate drug exposure for susceptible bacteria in a 250 kg patient with normal renal function. The case suggests the usefulness of therapeutic drug monitoring of antibiotics, especially in critically ill patients.