The ACG (Adjusted Clinical Groups) case-mix system is a classification method of diseases of patients, focused on the person. Depending on the pattern of these morbid conditions, the ACG system assigns each patient to a single group (an ACG group), which allows to capture the effects of a group of diseases in estimates of resource use. Diseases are classified into a diagnostic group (ADG) according to 5 clinical dimensions: duration (acute, recurrent or chronic), severity (minor/major vs stable/unstable), diagnostic assessment (symptoms vs diseases), etiology (infectious, traumatic or other), specialty (medical, surgical, obstetric, ...). All diseases can be classified into these dimensions and into one of 32 groups. The ACG case-mix system uses an algorithm to classify each patient into one of 93 ACG categories. Each person is assigned to an ACG according to his ADG combination, his age and his gender. With the repayment system "case-mix", surgery has become central for all great hospitals in virtue of its great productive potential. The case-mix index is one of the factors which influence the duration of hospitalization. The case-mix system has emphasized the importance of the duration of hospitalization, encouraging the planning of programs in order to discharge patients early after surgical operations. It has also stimulated the surgical activity in operating units with "budget" forecasts in which resources are provided according to an expected level of specialist surgery.