OBJECTIVE: To assess the impact of clinical guidelines to improve appropriate use of routine laboratory tests and bedside chest radiographs in a medical intensive care. DESIGN: A two-year (Period-1: 2005, Period-2: 2006), retrospective, comparative study, before and after policy implementation. PATIENTS: All consecutive patients admitted during the study periods. SETTING: A university hospital 15-bed medical ICU. INTERVENTION: Multifaceted intervention combining a daily routine prescription help-guide developed by a multidisciplinary group and displayed at patient's bedside, educational sessions and feedbacks by information on volumes of prescription. Individual adaptation to patient's clinical status was allowed by protocol. ASSESSMENT: The overall number and cost of laboratory tests and chest radiographs during Period-2 (with the help guide; from 01 to 12-2006) were compared to Period-1 (from 01 to 12-2005). RESULTS: Patients' general characteristics were similar during the two periods. A relative reduction of routine laboratory tests performance was observed per patient-ICU-day, ranging from 38 to 71.5% depending on the type of tests (P < 0.001 in all cases). For chest radiographs, a 41% relative reduction was observed between the two periods (P < 0.001). Daily ICU laboratory tests and chest radiographs cost per patient decreased from 114 to 56 euros. An overall 300,000 euros ICU cost reduction was directly related to the protocol implementation. CONCLUSION: The implementation of a laboratory tests and chest radiographs prescription protocol within our ICU induced an important cost saving.