OBJECTIVES: To measure the efficiency of pharmacy resources utilization in five Primary Care centres by the retrospective application of Adjusted Clinical Groups (ACG) in a usual clinical practice setting. METHODS: Retrospective study carried out on the basis of the clinical records from all the attended patients along the year 2006. Main variables: age, sex, case-mix/episodes, visits, pharmacy costs, centre, physician and service (Family Medicine or Pediatrics). ACG grouper (Starfield and Weiner, Johns Hopkins University, n=106) classifies each patient in a unique category of similar resource consumption. A Receiver Operating Characteristics (ROC-area under the curve) analysis was done to assess the predictive value of the model. The Efficiency Index (EI) was obtained as the quotient between the observed and the expected pharmacy costs according to ACG distribution (indirect standardization). The statistical package SPSS was used (p<0.05). RESULTS: A total of 80,775 patients were included. Average number of episodes 4.8±3.5 and visits 7.9±8.2; mean age: 40.7±22.9 years; and 46.9% of males. Intensity of utilization: 72.4%. Costs of drug prescription: €22.7 millions (55.6% of the total costs). Mean cost was €281.05±627.85. ROC curve analysis for episodes: 0.588 (p=0.000); sensibility: 37.3%, specificity: 73.1% and intra-class correlation coefficient C: 0.732 (p<0.001). The EI for each centre was respectively: 0.91 (CI: 0.78-1.04), 0.93 (CI: 0.77-1.09), 0.96 (CI: 0.81-1.11), 1.06 (CI: 0.95-1.17) and 1.07 (CI: 0.95-1.19), p<0.0001. Furthermore, differences between family physicians and pediatricians were observed (range: 0.55-1.46), p<0.0001. CONCLUSION: Results show a wide variability in the costs of pharmacy within centres and physicians. ACG provide an adjusted approximation to efficiency in pharmacy costs. Efficiency must not be considered as an isolated dimension of quality. The determination of the El could lead to a better knowledge of the prescription profile from individual physicians and/or primary care teams.