Despite CPOE (Computerized Physician Order Entry) systems' potential to enhance patient safety by reducing medication errors, recent studies have cast some doubts on their role in error reduction. CPOE systems with poorly designed interfaces have proven to cause users dissatisfaction and to introduce new kind of errors in the ordering process, suggesting a threat instead of an enhancement of patient safety. The main objective of this study is to identify usability problems related to a CPOE medication system's design and determining their severities. Two experts completed a cognitive walkthrough (CW) of an ordering task based on a clinical scenario for ordering the consolidation phase of chemotherapy for a leukemic patient. Fifty five usability problems were found and classified into eleven categories. CW identified cosmetic to catastrophic problems leading to inefficient use of the CPOE system and potentially resulting in users' confusion, longer ordering duration, and medication errors. The complexity of the CPOE design, its rigidness and lack of user guidance suggests the necessity to redesign the current user interface in order to match clinicians' ordering behaviors and to fully support them in the medication ordering process.