Cefdinir is a new, extended-spectrum, orally active, third-generation cephalosporin that is resistant to bacterial beta-lactamase production. To evaluate efficacy and safety of the antibiotic in maxillary sinusitis, its use was compared with amoxicillin/clavulanate (amox/clav), which is a well-accepted beta-lactamase-resistant antibiotic. In this investigator-blinded multicenter phase III clinical study, 569 patients were randomly assigned to one of three treatment regimens: one daily dose of cefdinir 600 mg (OD), cefdinir 300 mg every 12 h (BD), and amox/clav 500/125 mg every 8 h. All antibiotics were administered orally for 10 days. Maxillary sinusitis was documented by typical clinical signs and symptoms and was confirmed by X-ray imaging. Before treatment, the genus and species of any pathogens were determined from sinus aspirates. Cultures were tested for beta-lactmase production and in vitro resistance to cefdinir and amox/clav. The effectiveness of antibiotic treatment was evaluated 7-14 days after therapy and whether or not recurrent clinical symptoms or persistent infection was determined 21-35 days post-therapy. The appearance of any adverse events was classified as associated or not associated with the medication of the study. Present findings showed that the in vitro susceptibility of pathogens to cefdinir and amox/clav was similar. Cefdinir OD or BD was therapeutically as effective as or better than amox/clav, although cefdinir BD was not as useful as amox/clav clinically. Cefdinir OD and BD and amox/clav were well tolerated. The statistical incidence of adverse events was the same among the three treatment groups, although cefdinir OD treatment had significantly fewer treatment discontinuations due to adverse events than BD and amox/clav.