Background: The increasing resistance of bacteria to antibiotics threatens the survival of patients and health costs. Aim: To determine the impact of an antimicrobial stewardship program in bacterial resistance and antibiotic consumption. Materials and Methods: Quasi experimental study in a third level clinic in the city of Medellin, that evaluate in two time periods (pre-intervention between October 2012 and September 2013 and post intervention between October 2013 and September 2014) the impact of an antimicrobial stewardship program in terms of antibiotic consumption and bacterial ecology. Results: Adherence to institutional guidelines for management of infections in the post-intervention period was 82%. Antibiotic consumption of meropenem, ceftriaxone, vancomycin and colistin decreased, and imipenem, daptomycin and linezolid was increased. A significant decrease in antibiotic resistance in Pseudomonas aeruginosa was observed, including carbapenems. An increase of extended spectrum beta lactamase production (ESBL) in Enterobacteriaceae (especially E. coli) and piperacillin/tazobactam resistance was observed. Conclusions: The construction and implementation of a strategy in hospitals with guidelines for managing infectious diseases, restrictions in antibiotic use, a permanent monitoring system for the formulation of antibiotics, achieved a positive impact on reducing antibiotic use and bacterial resistance.