Handwashing is the single most important procedure in the prevention of nosocomial infections and yet it remains the most violated of all infection control procedures. With a sequential intervention study in an intensive care unit we have demonstrated that poor handwashing practices are associated with a high nosocomial infection rate, whereas good handwashing practices are associated with a low nosocomial infection rate. An educational and enforcement program designed to improve handwashing procedures can significantly reduce endemic nosocomial infection rates.