The two rigorous reports in this issue by investigators at the Centers for Disease Control and Prevention (CDC) make for exciting reading.',2 These two reports demonstrate the importance of shoe-leather epidemiology, highlight the power of the new molecular biology with its subtyping capabilities, and deal with the drama of a disease that has, in the two hospitals described, a mortality of 35% to 40%. When one reads Kool et al's description of the intense attempts to track down the source of the organism and the heroic effort to disinfect the water supply, it is no wonder that legionnaires' disease continues to command such attention 22 years after the outbreak at the 1976 American Legion Convention. Are there any new lessons to be gleaned from these two reports? After all, the major conclusions in these two reports have been reiterated in numerous publications and are standard knowledge in medicine textbooks: they include the link to water distribution systems, the link to intubation and the possibility of aspiration; the recognition that the clinical presentation of legionnaires' disease is nonspecific, such that special laboratory tests are needed; and the high risk for contracting legionnaires' disease in transplant recipients and in patients administered corticosteroids. However, there are a number of salient points that illustrate the need for prevention, and I will focus on them. What is remarkable to me is the number of outbreaks of hospital-acquired legionnaires' disease that still are occurring. How can this be after so many reports, dating back to 1982, have linked the contamination of water distribution systems by Legionella to subsequent discovery of legionnaires' disease on the hospital ward? More reports with the same theme will be published in 1999. The epidemiological version of Koch's postulates3 have been fulfilled for linking Legionella colonization in hospital water supplies to hospital-acquired legionnaires' disease.4,5 (Koch's postulates have not been fulfilled for linking cooling tower reservoirs to legionnaires' disease). Since the reservoir for dissemination has been so well established, as in the two CDC reports in this issue, why have measures not been enacted to prevent this disease? Pittsburgh investigators have recommended routine environmental cultures of the hospital water supply since 1986, so as to screen for the possibility of occult legionnaires' disease in the hospital (Figures 1 and 2).6 Obtaining surveillance cultures of water distribution systems in hospitals performing transplants is mandatory, in our opinion.7,8 This is a proactive, scientifically based approach that is inexpensive and focuses attention on those patients and hospitals at the highest risk. The two CDC articles both conclude that intensive surveillance for hospital-acquired legionellosis is necessary for prevention. Conspicuous by its absence is the alternative approach suggested by Pittsburgh investigators and mandated by the Allegheny County Health Department guidelines for prevention of Legi'onella infections9 (Figures 1 and 2). I am concerned that to perform patient surveillance without environmental cultures is to ignore the epidemiological data that have been collected by so many investigators, including the CDC. Knowledge of the reservoir for the organism can be applied to prevention! It makes little sense to me to initiate environmental cultures after cases of hospital-acquired legionnaires' disease have been discovered, as recommended by the CDC. Lepine et al make the telling points that hospital-acquired legionel-