Five patients with extensive deep burns developed septicaemia due to Pseudomonas aeruginosa serogroup O-7.8 and phage type 21 or 21/188 shortly after they had been admitted to hospital. Four other burned patients became colonized with the same strain. The source of infection was contaminated tap water used for irrigation of the burns, as part of the first-aid treatment which the patients received when entering the hospital. Contamination was restricted to showers and tubing that were permanently connected to the taps, and the outbreak stopped after they had been disinfected. Tubing and showers used for irrigation of burns should be dismantled and heat-disinfected after each patient and not reconnected to the taps until immediately before the next treatment. Taps used for irrigation of burns should be monitored regularly for the presence of P. aeruginosa and other potentially pathogenic bacteria. Routine typing of P. aeruginosa isolates from burned patients is indicated in order to detect and eliminate hidden sources of infection.