Research suggests that more than half of all emergency department (ED) visits in the United States are for nonurgent conditions, leading to billions of dollars in potentially avoidable spending annually. In this study, we examine the effects of co-payment changes on ED utilization among children enrolled in ALL Kids, Alabama's Children's Health Insurance Program We separately model the effect of the 2003 co-payment increases on the monthly probability of any ED visit, and visits within three severity categories, using linear probability models that control for beneficiary characteristics and time trends that are allowed to vary in the pre- and postperiods. We observe a small decline in the probability of ED visits 1 year after the co-payment increase. However, low-severity visits, which we hypothesize to be more price sensitive, show no significant evidence of a decline. Our study suggests that the modest co-payment changes were not effective in improving the efficiency of ED utilization. © The Author(s) 2013.