A continuous observation time study was used to track 1,568 patients through various stages of emergency department care in order to identify sources of delay. Patients initially were assigned to one of four categories of decreasing acuity, and flow profiles were compared for each category. Patients with lowest acuity level experienced long delays in moving through the ED, although the actual evaluation and treatment time was brief. Urinalyses, procedures, radiographs, and blood tests had an increasing impact on treatment times (45 minutes, 63 minutes, 65 minutes, and 126 minutes, respectively, compared to 31 minutes with no tests or procedures). Patients of increasing acuity moved more quickly, but with a longer evaluation and treatment time. The most critical patients moved most quickly but with a brief evaluation and treatment time due to a rapid disposition from the ED or death. This relationship suggests an emergency care system that is oriented toward the efficient care of high-acuity patients but that is less effective for low-acuity patients. This occurs despite the fact that low-acuity patients comprise the vast majority of the ED census, and represent a group for whom delay is a frequent source of patient dissatisfaction.