This paper describes an innovative approach to the evaluation of a handheld prescription writing application. Participants (10 physicians) were asked to perform a series of tasks involving entering prescriptions into the application from a medication list. The study procedure involved the collection of data consisting of transcripts of the subjects who were asked to "think aloud" while interacting with the prescription writing program to enter medications. All user interactions with the device were video and audio recorded. Analysis of the protocols was conducted in two phases: (1) usability problems were identified from coding of the transcripts and video data, (2) actual errors in entering prescription data were also identified. The results indicated that there were a variety of usability problems, with most related to interface design issues. In examining the relationship between usability problems and errors, it was found that certain types of usability problems were closely associated with the occurrence of specific types of errors in prescription of medications. Implications for identifying and predicting technology-induced error are discussed in the context of improving the safety of health care information systems.