OBJECTIVES: In 1997, the Belgian reimbursement for prophylaxis in surgery changed from FFS to a lump-sum per operation. We investigate whether this altered the medical practice w.r.t. the administration of prophylaxis. METHODS: The entire population that had undergone one of these six operations was included: endoscopic resection of prostate, surgical treatment of discal hernia, arthroplasty of hip with total prosthesis, reconstruction of cruciate knee ligaments by arthroscopy, total hysterectomy (abdominal) and total hysterectomy (vaginal), accounting for 159981 operations and a cost for antibiotics of €10853 million. Cluster analysis is performed to identify outliers, classify cases/patients with similar antibiotic treatment, compare both periods, and condense information. Different treatment patterns emerging from the cluster analyses were examined to assess the appropriateness of the administered products. RESULTS: A drastic and immediate change in medical practice: 11% of the patients did not receive antibiotics anymore contrasting with the situation before, where all hospitals administered antibiotics to all patients. Large differences between hospitals can be detected. Both average number of DDD and costs dropped sharply. Cluster analyses indicate that less products are used and that the number of units administered is smaller. Concerning antimicrobial spectra, especially the use of second-generation cephalosporines was reduced substantially. Further, a large share of the prophylaxis is still not administered according to (international) guidelines. A distinction is made between administering too much of an appropriate product and administering an inappropriate product. The following percentages indicate the share of patients that receive prophylaxis in accordance with good medical practice: prostate: 54%; hernia:81%, hip:50%, cruciate ligaments:66%, abdominal hysterectomy:78% and vaginal hysterectomy:67%. CONCLUSIONS: Both goals of the new mechanism (reducing use and variety of antimicrobial products in order to contain the threat of antibiotic resistance and a reduction in costs) seem to be accomplished. However, prophylaxis is still administered injudiciously and inappropriately to a large share of the patients.