BACKGROUND Incorporating in the Intensive Care Unit (ICU) a clinical pharmacist who performs interventions on antimicrobials may be cost-effective. OBJECTIVE To evaluate the clinical and economic impact of clinical pharmacist interventions on antimicrobials in an ICU. To identify drug related problems and medication errors detected by the pharmacist. METHODS A retrospective observational study was performed to analyze drug related problems, medication errors and clinical pharmacist interventions related to antimicrobials in adults admitted to an ICU in a 5-month period. The economic impact of pharmacist interventions was estimated considering difference in cost derived from antimicrobial treatment, adverse drug events and clinical pharmacist time. RESULTS A total of 212 drug related problems were detected in 114 patients, 18 being medication errors. Clinical pharmacist developed one intervention for each problem identified. 204 interventions (96.2%) were considered important with improved patient care and 7 (3.3%) very important. No negative impact of any intervention was identified. Physicians accepted 97.6% of the interventions. A potential saving of 10,905 € was estimated as a result of pharmacist interventions and 4.8 € were avoided per euro invested in a clinical pharmacist. CONCLUSIONS A clinical pharmacist performing interventions on antimicrobials in the ICU has a positive impact on patient care and decreases costs.