Between March and May 1996 Stenotrophomonas maltophilia was cultured from endotracheal aspirate samples from five preterm infants in a neonatal intensive care unit (NICU). Four infants were superficially colonized, but a fifth died due to S. maltophilia septicaemia. S. maltophilia was cultured from tap water from three outlets in the NICU including one with a previously unnoticed defective sink drain. Water from these outlets was used to wash the preterm infants. Environmental and clinical S. maltophilia isolates yielded identical banding patterns on random arbitrary polymorphic DNA (RAPD) PCR analysis. The outbreak was controlled by reinforcement of hand disinfection, limitation of the use of tap water for hand washing and by using sterile water to wash the preterm infants. We conclude that tap water should not be used for washing preterm infants in the NICU, unless steps are taken to prevent microbial growth in the outlets.