Abstract Patient Safety Leading Article Series, 2004. This leading article continues our series on the general topic of patient safety. In the last issue of BJS the dysfunctional professional relationship was highlighted and the way the airlines have tried to counter the associated risks. The present article examines patient risk from an entirely different viewpoint. Nosocomial infection has been with us ever since hospitals were invented but there are problems particular to the modern era. Professor David Leaper comments on today's risks.