Summary An outbreak of multidrug-resistant Klebsiella pneumoniae producing the extended-spectrum β-lactamase CTX-M15 affected 247 mainly elderly patients in more than 30 wards in a 1000-bedded swedish teaching hospital between May 2005 and August 2007. A manual search of the hospital administrative records for possible contacts between cases in wards and outpatient settings revealed a complex chain of transmission. Faecal screening identified twice as many cases as cultures from clinical samples. Transmission occurred by direct and indirect patient-to-patient contact, facilitated by patient overcrowding. Interventions included formation of a steering group with economic power, increased bed numbers, better compliance with alcohol hand disinfection and hospital dress code, better hand hygiene for patients and improved cleaning. The cost of the interventions was estimated to be €3 million. Special infection control policies were not necessary, but resources were needed to make existing policies possible to follow, and for educational efforts to improve compliance.