The aim of this study is to evaluate the relationship between comorbidity and the disability in patients referred to a rehabilitation centre. Patients often suffer from associated diseases which may negatively influence the outcome.After evaluation of several scales, the Cumulative Illness Rating Scale (CIRS) was selected. For the study 413 consecutive patients were enrolled.In this series, neurological patients presented with higher severity and higher comorbidity than orthopaedic patients. Orthopaedic cases were older but neurological cases had higher severity and higher comorbidity. Indeed, a significant negative correlation between severity and dependence scores was found. Comorbidity, also, showed a significant negative correlation with FIM both in the orthopaedic and in the neurological patients. These data show that severe comorbid conditions influence functional autonomy. Severity and Comorbidity were correlated both in orthopaedic cases and in neurological cases as well.A review of geriatric literature demonstrates lower values in the patients in rehabilitation. The difference is due to our accurate selection of patients at admission, where general health conditions are considered. In conclusion the CIRS scale should be used as a method for selecting patients at admission or as a prognostic index for improvement at discharge. CIRS, however, has some inconveniences and amelioration is necessary, such as the inclusion of a double testing (admission-discharge), psychiatric disturbances and a new item for skin alone. CIRS should not be applied if a patient referred for rehabilitation has the worst score in even a single item.