Nosocomial infection rates, as determined by either incidence or prevalence methods, are considered important data in infection control programs. Many factors besides infection control measures affect infection rates— eg, illness acuity of the patient population. However, there is evidence that when these factors remain constant, a lowering of the infection rate can be the result of infection control efforts. We wish to illustrate how a dramatic drop in infection rate may mislead infection control personnel into a false sense of accomplishment, when in reality it is an effect of changing medical practices.