An outbreak of six cases of hospital-acquired Pseudomonas aeruginosa infections (two pneumonia two septicaemia, two skin/wound infection) occurred between August and September 2000 in an adult haematology–oncology unit at a tertiary-care centre. During the outbreak, hospital-acquired infection (HAI) incidence density rates rose from 29.4 to 62.3 (P<0.05) infections per 1000 days at risk (i.e., neutropenic days). A systematic outbreak management system was actioned in accordance with a German draft guideline. Multiple samples from the patients' environment were tested for the presence of P. aeruginosa. A total of 4.5% of samples from sanitary equipment and 20.0% of samples from surface cleaning equipment were found to be contaminated with P. aeruginosa. Genotypic analysis by pulsed-field gel electrophoresis showed different patterns for all (N=6) of the patient isolates, however, two of the patient isolates were identical in comparison with environmental isolates from cleaning equipment (four samples) and sanitary equipment (one sample). Our investigation revealed that the cleaning staff had used cleaning solution instead of disinfectants for decontamination of the patients' environment. The outbreak was terminated after re-adoption of surface disinfection, application of sterile filters on taps and shower heads, chemical disinfection of the washbasin drains, and appointment of a hospital hygiene nurse to a previously unfilled position. After institution of the control measures, HAI incidence densities decreased to pre-outbreak level. This investigation emphasizes the need to carefully evaluate cleaning and disinfection practices for patient care, particularly in neutropenic patients.