The aim of this study was to discuss about prevalence and clinical rilevance of comorbidity in the elderly patients. Our sample included 2373 (mean age 77.8 +/- 8.5; 1302 men and 1071 woman) consecutively admitted to the University Department of Geriatric Medicine of Torino. We examined some demographic variables, cognitive and functional status, main pathologies. Severity of illness was assessed using the C.I.R.S. The coexistence of two or more diseases was 83%: cardiovascular and chronic pulmonary diseases were the most frequently recorded (respectively 68% and 27%). The comorbidity and severity indexes of C.I.R.S. were associated respectively with mortality (O.R. 1.78; C.I. 1.36 - 2.33) and length of hospital staying (O.R. 2.35; C.I. 1.19 - 4.65). Comorbidity is an important specific prognostic indicator for reliable risk stratification of older patients.