Five cases of nosocomial infection caused by Serratia marcescens were traced to backflow of blood from nonsterile evacuated blood-collection tubes. The mechanism of backflow was investigated theoretically and the conditions were determined under which backflow can occur. The theory was confirmed by experiments conducted on a simulated venous system and by measurements of the venous pressure in the brachial vein of a patient during application and removal of a tourniquet. Various possible solutions to the backflow hazard include strict adherence to correct venipuncture technique, sterilization of all blood-collection tubes, improvement of the vacuum in the tubes and incorporation of a check valve into the system.