Personal Health Records (PHRs) and patient portals are becoming more popular as health systems continue to emphasize patient centered health care delivery. Our study investigates the factors that influence adoption and utilization of PHR systems using a mixed method approach. Our data include PHR system utilization data collected about registered users of seven primary care physicians during the period between September 2009 and February 2011, as well as interviews of a subset of these users. Analyses of these data provide insights into how and why users interact with the system. They also assist in identifying ways to enhance the overall PHR portal experience for physicians and patients. In this paper, we present our preliminary findings and discuss potential future research directions.