In order to determine the clinical impact of Klebsiella bacteremia on critically ill patients, a matched cohort study was conducted between January 1992 and December 2000. During the study period, all intensive care unit (ICU) patients with nosocomial Klebsiella bacteremia were defined as cases (n=52), but two of these patients were excluded from the matched cohort due to incomplete medical records. The remaining 50 patients were matched at a ratio of 1:2 with control patients (n=100) on the basis of the APACHE II severity of disease classification system. Patients with Klebsiella bacteremia experienced acute renal failure and hemodynamic instability more often than controls. They also had a longer ICU stay and longer ventilator dependence. In-hospital mortality rates for cases and controls were nearly equal (36% vs. 37%, respectively; P=0.905). In conclusion, after adjusting accurately for severity of underlying disease and acute illness, no difference in mortality was found between ICU patients with Klebsiella bacteremia and their matched control subjects.