A retrospective case-control study was initiated at Uppsala University Hospital in 2006 during a major outbreak caused by a Klebsiella pneumoniae strain producing CTX-M-15. To identify risk factors associated with acquisition of the outbreak strain in the urinary tract, 52 case patients with a urine culture positive for the outbreak strain between 1 May and 31 December 2005 were enrolled. Case patients were matched 1:2 with concurrently hospitalized control patients with significant growth of susceptible Escherichia coli in a urine sample. Conditional logistic regression analyses identified hospital stay >or=9 days (odds ratio (OR) 18.8, 95% confidence interval (CI) 5.74-61.2), nasogastric feeding tube (OR 18.0, 95% CI 2.28-142) and diarrhoea (OR 9.62, 95% CI 3.30-28.1) as risk factors with high ORs. The odds of previous use of cephalosporins were 7.58 (95% CI 3.13-18.4) times higher in case patients compared with the controls. Several multivariable models were evaluated to reduce bias from confounding. These models identified prolonged period of hospitalization, diarrhoea, malignancy and antibiotic use as the most important risk factors for acquisition of the outbreak strain, factors that are often found in elderly patients with a poor functional status.