In our health care system, a medical treatment is developed that helps people well for a while. After a few years, new insights and lessons learned lead to a new discovery or an alternative treatment. At that moment it becomes challenging, because then the question is asked: how much does the alternative treatment cost? Is it more expensive than what we do today? OPAT stand for ‘Outpatient Parenteral Antimicrobial Therapy’ and this is an example of an alternative medical treatment where the cost is unclear in the Belgian setting. Primarily, intravenous antimicrobial treatment takes place exclusively in the hospital, whereas with OPAT, the start of the treatment is in the hospital after which treatment is continued at home. This is interesting for the patient, who generally prefers to recover at home rather than the hospital. In fact, research shows that OPAT is as safe and effective as hospital treatment. But what if the cost of OPAT is more expensive for the patient or the health insurance? Is it then smart to offer OPAT as an alternative? Determining costs, puzzling, and calculating This study started by looking at cost items so that it was clear what was being charged for treatment. There are costs for admission, nursing care, medication, etc. With OPAT, part of the treatment continues at home with the help of a home care nurse. Thanks to the social security in Belgium, there is one big pot of money that is distributed between the hospital, home care, elderly care, etc. That pot is not infinitely deep which makes it necessary to think about how we distribute each Euro. A patient also pays a part. In order to know how much more expensive OPAT is than hospital treatment, it is necessary to put together a large financial puzzle with, as pieces of the puzzle, the different costs that are charged to the patient on the one hand, and to the health insurance on the other. Once it was clear what was charged to whom, a result for each treatment could be calculated for the patient and for the health insurance in an OPAT setting and a hospitalization setting. What was also examined was the difference in cost between two Belgian hospitals. Conclusion So what is the conclusion: is OPAT more expensive or cheaper than full treatment in the hospital? The answer is typically Belgian. Sometimes OPAT is less expensive, sometimes it is not. It appears to be so for both the patient and the health insurance. Much depends on the medication used.