OBJECTIVES: To estimate the daily cost of intensive care unit (ICU) stay in France using a microcosting methodology. METHODS: A multicentre prospective cost analysis study was carried out among 23 French ICUs randomly selected from the French National Hospital database stratified by hospital category (regional, university and private non profit). Each ICU enrolled 5 adult patients admitted from May to October, 2009, selected at random, with a simplified acute physiology score (SAPSII)≥15 at admission and with at least 1 reanimation medical act. All healthcare resources used by each patient over a 24-hour period were recorded, as well as the time spent by all hospital staff involved in the patient’s management. All resources identified were valued from a hospital perspective (reference year 2009) based on unit cost data provided by each centres. Bi-variant analyses were carried out to identify potential cost-drivers. RESULTS: A total of 104 patients were enrolled by 21 ICUs (14 polyvalent, 3 surgical and 4 medical) were included. The mean age of patients was 62.3 years (SD 14.9); 64% were male; 86% were mechanically ventilated and the median Sequential Organ Failure Assessment (SOFA) score was 6 (SD 4.3). The average daily cost of ICU per patient was €1,424 (SD €520). Staff time represented the largest component of this cost (43%) followed by overheads, capital, hotel and nutrition assigned to the ICU (22.9%). Medication and consumables used accounted for 18.6% of the total cost. The majority of the cost (59%) was patient-dependant. The two main patient-dependant factors associated with significantly higher costs were: a high SOFA score and being on continuous mechanical ventilation. CONCLUSIONS: This first French microcosting study in ICU demonstrates that the cost per day of ICU care is substantially depends on the patient’s medical profile and mainly driven by labour components.