Overly simplistic or complex user interfaces of Computerized Provider Order Entry (CPOE) systems may impede clinical efficiency and the decision-making process by increasing cognitive load and diverting physicians' attention from the clinical task at hand. To design better healthcare user interfaces there is an imperative need for developing new evaluation methodologies to quantify the cognitive complexity of CPOE user interfaces. The purpose of this study is to propose a method for quantitative measurement of cognitive complexity of CPOE user interfaces; to validate the developed metrics by characterizing and comparing the cognitive complexity of three CPOE systems; and to gather preliminary usability data that can be correlated to quantitative estimations of cognitive load of user interfaces.