OBJECTIVE To describe an outbreak of multidrug-resistant Pseudomonas aeruginosa in which the hospital waste-pipe system was the likely source of contamination and to report the bundle of measures that facilitated the long-term control of the outbreak. DESIGN Outbreak investigation. SETTING The hematology unit of a tertiary-care referral center. PATIENTS Patients who were colonized or infected with P. aeruginosa belonging to the clonal outbreak. METHODS Patients admitted to our 15-bed stem-cell transplantation hematology unit were screened for P. aeruginosa carriage. Pseudomonas aeruginosa isolates were also obtained from diagnostic samples. We assessed the microbiological contamination of P-traps, water and toilets for 42 months. Extended-spectrum β-lactamases (ESBLs) and metallo-β-lactamases (MBLs) were screened and identified by polymerase chain reaction (PCR) and sequencing. Molecular typing of ESBL- or MBL-producing isolates was carried out using pulsed-field gel electrophoresis (PFGE) and multilocus sequence typing (MLST). RESULTS From 2009 to 2013, a biclonal outbreak of IMP-19-producing ST235 (11 cases) and IMP-29-producing ST111 (10 cases) of P. aeruginosa occurred. The environmental investigation strongly suggested that P-traps were the reservoirs for the outbreak strains. A bundle of infection control measures, including engineering interventions on water outlets and disinfection of P-traps, controlled the outbreak. CONCLUSIONS We report a prolonged outbreak of IMP-producing high-risk clones of P. aeruginosa, for which P-traps seems to play a major role in cross-transmission. It appears essential to implement proactive measures to limit the bacterial load in water fittings of high-risk units. Infect Control Hosp Epidemiol 2018;39:164-169.