Health information exchange (HIE) systems integrate patient-level data that originates in several health information systems (HISs). As these systems bridge information gaps, they are expected to improve the caregiving process in terms of resource utilization and quality of care. Nevertheless, effective use of the system is a hurdle in realizing these benefits fully. Understanding actual individual use of the system is therefore of great importance. Several studies described factors that result in the use of the system, yet seldom examined their affect on characteristics of use. Furthermore, few studies empirically evaluated the association between the characteristics of actual individual use and decisions made during treatment. In this study, we intend to identify the antecedents of pattern of system use, namely patient-related, user-related, and environmental factors. The pattern of use is described by several variables. We then explore the link of the pattern of use and potential antecedents to clinical decisions, specifically the unit to which patients were admitted and the number of ancillary tests ordered. Our hypotheses are tested using clinical data and HIE log files of an emergency department (ED) in a large Israeli hospital.