This study analyzes one component of the health care safety net to determine whether or not being enrolled in a free or low-cost primary care physician access program subsequently affects emergency room utilization by uninsured adults ages 18 through 64. The study is a quantitative analysis of more than 40,000 individual patient records. An Intensity of Use Indicator (IUI) was developed for tracking individual and group ED utilization trends. The IUI should prove useful to hospital and other not-for-profit organizations concerned with tracking cost effectiveness of programs for uninsured adults.