Too often, online searches for health information are time consuming and produce results that are not sufficiently precise to answer clinicians' or patients' questions. The PERSIVAL project is designed to circumvent this problem by personalizing and tailoring searches and presentation to the demands of the user and the particular clinical context. This paper focuses on a cognitive evaluation of one component of this project, a Query User Interface (QUI). The study examines the system's ability to allow users to easily and intuitively express their information needs. We performed several analyses including a cognitive walkthrough of the interface and quantitative estimations of cognitive load. The paper also presents a preliminary analysis of usability testing. The analyses suggest that there are features in the QUI that contribute to a greater cognitive load and result in greater effort on the part of the subject. The results of usability testing are consistent with these findings. However, subjects found it to be relatively easy and intuitive to generate well-formed queries using the interface. This study contributed to the iterative design of the interface and to the next generation of the PERSIVAL system.