OBJECTIVES:   The objective of the study is to obtain behaviour of the cost’s relative average weights of the assistance with the retrospective application of the Adjusted Clinical Groups (ACG’s) in 16 teams of Primary Care with an attended population in the clinical practice use. METHODS: Multicentre, retrospective study based on electronic records of patients seeking care during 2008 in the regions of Aragon, Balears and Catalonia. Main measurements: universal variables (age, sex, health service-family practice/paediatrics) and dependent variables: episodes and total cost (visits, diagnostic test, referrals, drugs). The ACG case-mix System software (version 7.1; N=106) classified subjects into a single category for a given annual resource consumption. The model of cost per each patient was established differencing the fix cost and the variable. Outlier patients were considered those surpassing T= Q3+1.5(Q3-Q1)=€1,778.6 for total cost expenditure. Log transformation of the dependent variable was carried out to reduce skewness of the distribution and make it close to normal. Explanatory power was calculated by coefficients of determination (R2). Statistical software: SPSS, p<0.05. RESULTS: The total number of the studied patients was 227,235 (intensity of use: 75.6%), with an average 4.5±3.2 episodes. The age average was of 44.1±23.7 years,  56.6% women (13.5% paediatrics). The distribution of costs was €148,657,137. The total unitary cost per patient/year 654.2±851.7€ (relative weights of reference). Patient’s case-mix: 57.2% of the study population was grouped into 10 ACG. The explanatory power of the ACG classification system was 36.3% (Ln: 41.2%), p<0,001. 6.2% of patients were considered Outliers (N=14.066). It details the form skewness and the average relative weights per each category of ACG’s classification. CONCLUSIONS: The ACG are an acceptable system of classification of patients in situation of clinical practice use. Some ACG classification categories should be separeted due to the high outliers number.