This paper looks at the use of Health Information Systems (HIS) from a communication perspective. Drawing on Niklas Luhmann's systems theory, we analyze patients' self-observations in relation to their disease, as well as physicians' observations of PRO data collected via a mobile application. Based on the analysis, we argue that patient-physician miscommunication occurs, and that the premises of HIS supported information sharing rest on a too simplistic conceptualization of communication. Conclusively, we discuss the implications for the use of HIS instruments in support of patient-physician information sharing, communication, and understanding.