OBJECTIVES To investigate the association of different chronic comorbidities, considered singularly and together in Cumulative Illness Rating Scale (CIRS) indexes, with pneumonia diagnosis in a group of elderly frail hospitalized patients. DESIGN AND METHODS With a retrospective cohort design, all clinical records of frail (Rockwood ≥ 5) nonterminal patients ≥ 65 years old acutely admitted over a 8-month span in an internal medicine ward were evaluated. Pneumonia status and its categorization (community-acquired, CAP, vs healthcare-associated, HCAP) were defined according to chest radiology findings and validated criteria. Chronic comorbidities, CIRS Comorbidity Score and CIRS Severity Index were collected for each participant through a standardized methodology. Multivariate logistic regression models were applied to assess the association of each comorbid condition or scores with pneumonia. RESULTS 1199 patients (546 M, median age 81.9, IQR 72.8-87.9 years), of whom 239 with pneumonia (180 CAP, 59 HCAP) were evaluated. CIRS Comorbidity Score was significantly associated with pneumonia, both at an age- and sex-adjusted model and at a multivariate model (OR for each unitary increase 1.03, 95% CI 1.001-1.062, p=0.04), together with provenience from nursing home (OR 1.96, 95% CI 1.41-2.73, p<0.001). Among single comorbidities, only COPD (OR 2.7, 95% CI 1.9-3.6, p<0.001) and dementia (OR 2.3, 95% CI 1.7-3.3, p<0.001) were associated with pneumonia, while stroke, cancer, cardiovascular, chronic liver and kidney disease were not. CONCLUSIONS In a small cohort of elderly frail hospitalized patients, measures of multimorbidity, like CIRS, are significantly associated with the risk of pneumonia. COPD and dementia are the main conditions concurring to define this risk.