OBJECTIVE To determine which treatment strategy offers the most cost-effective option in managing acute sinusitis. DESIGN The modelling procedure included five clinical strategies, varying from "wait and see for a week", to "prescribing antibiotics selectively" to "prescribing antibiotics immediately" and to "performing further diagnostics". SETTING Outpatient clinics and primary health care. PATIENTS Data were derived from clinical trials that included otherwise healthy patients with acute sinusitis. MAIN OUTCOME MEASURES The marginal cost-effectiveness. RESULTS By using the strategy "wait and see for a week", 91.5% of the patients, by "prescribing antibiotics selectively", 93.2%, of the patients, and by "prescribing antibiotics immediately", 94.5% of the patients were cured after a 1-week period. The costs for curing one additional patient were Dutch Florin (DFL) 516 when antibiotics were selectively prescribed, and DFL 882 when antibiotics were immediately prescribed. Further diagnostic procedures did not improve outcome in terms of marginal cost-effectiveness. CONCLUSIONS In patients presenting with acute sinusitis, postponing antibiotics for 1 week is the most cost-effective strategy.