Background: There have been some studies on the usefulness of C-reactive protein (CRP) in diagnosis and as prognostic factor for community-acquired pneumonia (CAP), but there are few studies in geriatric patients. objective: was to determine the association of serum CRP and the response to antimicrobial therapy in elderly patients with CAP. Methods: We performed a prospective cohort study in the geriatric service at the ISSEYM Medical Center, Toluca. Serum levels of CRP were determined by immunoturbidimetric analysis (Roche, Hitachi model 9l7). results: We included 19 patients, mean age 80 ± 5.6 years, range 71 to 91, with an average follow-up of 10 ± 2 days (range 7 to 15), its distribution was 12 men (63%) and 7 women (37%). By multivariate analysis we found that subjects who had a serum level of CRP above 10 mg / dL after the treatment had a relative risk (RR) of 1.5 (95% CI, 0.67-3.33, P = 0.158) to have a torpid evolution. Conclusion: serum CRP> 10 mg/dL is an indicator of severity for CAP.