Rationale: Variation in practice and outcomes, not explained by patient or illness characteristics, is common in health care, including in intensive care units. Objective: To quantify within-intensive care unit, between-physician variation in resource use in a single medical intensive care unit. Methods: Prospective, noninterventional study in a medical intensive care unit where 9 intensivists provide care in 14 day rotations. Consecutive sample of 1184 initial patient admissions whose care was provided by a single intensivist. Multivariate models were constructed for average daily discretionary costs, intensive care unit length of stay, and hospital mortality, adjusting for patient and illness characteristics, and workload. Measurements and Main Results: The identity of the intensivist was a significant predictor for average daily discretionary costs (p<0.0001), but not intensive care unit length of stay (p=0.33) or hospital mortality (p=0.83). The intensivists had more influence on costs than all other variables except the severity and type of acute illness. Average daily discretionary costs varied by 43% across the different intensivists, equating to a mean difference of $1003 per admission between the highest and lowest terciles of intensivists. Conclusions: There are large differences among intensivists in the amount of resources they use to manage critically ill patients. Higher resource use was not associated with lower length of stay or mortality.