We examined a cluster of 5 hemodialysis patients who contracted gram-negative bacteremia. A nurse who used an artificial fingernail to open a vial of heparin that was mixed to make a flush solution had a culture of an artificial fingernail specimen positive for Serratia marcescens. The typing of the S. marcescens strains isolated from the 5 patients and the nurse showed them to be identical. This finding provides strong support for policies prohibiting artificial nails for healthcare workers in all hemodialysis units.