Background: We had previously observed that a large proportion of peripheral intravenous (IV) catheters placed in patients on a regular medical ward at our hospital were unnecessary. We conducted the current study to assess the effect of a quality improvement project led by medicine house staff on the prevalence of unnecessary peripheral IV catheters (those without any therapeutic use, referred to as idle). Methods: All patients on four regular-care medical wards of a large university-affiliated veterans hospital were included in the study. The proportion of IV catheter episodes in which catheters were idle 2 or more consecutive days (idle episodes) and the proportion of patients exposed to an idle catheter episode were determined by direct observation, chart review, and patient interview before and after a multidisciplinary quality improvement task force defined guidelines for appropriate IV catheter use and made recommendations for hospital policy changes related to IV catheter use. Results: The proportion of all IV catheter episodes that were idle catheter episodes decreased significantly after the intervention (42% before vs 29% after, P<.01), as did the proportion of patients with an IV catheter who had at least one idle IV catheter episode (43% vs 27%, P<.001). Conclusions: This quality improvement effort successfully reduced unnecessary IV catheter use. We suspect that house-staff involvement in the intervention was critical. We encourage other academic medical centers to involve house staff in quality improvement activities to improve patient care and to enhance the education of house staff regarding quality improvement processes.