This study describes an outbreak of Serratia marcescens and its investigation and control in a neonatal intensive care unit (NICU). During a three-month period, five infants were colonised or infected by a single strain of S. marcescens. A case-control study, culture surveys and pulse-field gel electrophoresis analysis implicated a bottle soap dispenser as a reservoir of S. marcescens (P=0.032). Infants with S. marcescens colonisation or infection were also more likely to have been exposed to a central or percutaneous venous catheter (P=0.05) and had had longer exposure to endotracheal intubation (P=0.05). Soap dispensers are used in many hospitals and may be an unrecognised source of nosocomial infections. This potential source of infection could be reduced by using 'airless' dispensers which have no air intake for the distribution of soap. Prompt intervention and strict adherence to alcoholic hand disinfection were the key factors that led to the successful control of this outbreak.