As part of a broader project to improve the usability of computerized physician order entry (CPOE) systems, we set out to study the cognitive tasks physicians undertake to write "admission orders" when admitting a patient to the hospital. In particular, we evaluate the hypothesis that physicians' mental model of diagnostic and therapeutic planning is problem based, whereas both paper-based ordering and CPOE are typically organized around functional categories of orders such as those reflected in the mnemonic ADCVAANDIML. A task analysis was performed which included think-aloud observations of physicians writing orders in clinical care settings and for fictional case-scenarios, as well as a semi-structured questionnaire. Our work finds core tasks of admitting a patient to hospital and conflicts between physicians' mental model and traditional ordering systems. Based on our study, we suggest improvements to traditional CPOE systems.