OBJECTIVES: To assess the degree of inadequacy of antibiotic utilisation and its associated costs in PHC centers in Catalonia. METHOD: We used an observational design. The universe of the study was all the visits due to infectious diseases to PHC doctors of the 40 basic health care areas of the Region “Costa de Ponent”. 2.500 visits were included in the sample, corresponding to 29 centers and 251 prescribers. The source of information was the clinical records. The adequacy of the treatment prescribed was assessed with reference to the recommendations of the Catalan Society of Family Medicine, adjusted to the pattern of antimicrobial resistance in the area. The efficiency of the prescription was assessed taking as a reference the minimum cost required by an adequate treatment that could be achieved by using the cheapest alternatives. RESULTS: The number of valid observations (visits due to infectious diseases) was 2,470. In 1,349 cases (54.6%) an antibiotic was prescribed. The treatment prescribed was adequate in 56.3% of the cases. The treatment was assessed as inadequate in 43.7% of the cases. In 92 cases an antibiotic was required and not prescribed and in 680 cases an antibiotic was not required but prescribed and in the remaining 294 cases an antibiotic was required, but the antibiotic prescribed was not the recommended one. The total cost of the antibiotics prescribed was 3.5 million PTA. of which 2.4 million (68.7%) could apparently have been avoided. 1.9 million corresponded to clinically inadequate prescriptions and the remaining 0.5 corresponded to clinically adequate prescriptions that used brands of higher price than the lowest priced alternatives available on the market. CONCLUSIONS: Inadequate use of antibiotic causes unnecessary costs to the health system and increases antibiotic resistance. Action should be taken to improve the adequacy and efficiency of prescribing habits.