Abstract: A system provides decision support information that improves patient care by enabling physicians who are placing orders to see historical treatment ordering patterns and adjust order associated workflow (e.g., as part of a computerized Health Care Order Entry System) based on an analysis of past orders placed on similar types of patients with similar demographic information under similar conditions. A patient treatment decision support system employing historical treatment order data includes a pre-processor. The pre-processor analyzes and collates historical patient treatment order information for multiple different patients based on criteria including diagnostic code to provide historical order information identifying treatment orders previously initiated for patients associated with a treatment order of a particular type. The historical treatment order information is stored by the pre-processor in a repository.