We performed a nested case-control study (ratio of 1:4) on the emergence of tigecycline-resistant multidrug-resistant Klebsiella pneumoniae (TR-MDRKP) isolates among patients who initially presented with a tigecycline-susceptible MDRKP isolate. Out of 260 patients, 24 (9%) had a subsequent clinical culture positive for a TR-MDRKP isolate within the 90-day follow-up period. On logistic regression analyses, receipt of tigecycline (adjusted odds ratio [OR], 5.06; 95% confidence interval [CI], 1.80 to 14.23; P ‫؍‬ 0.002) was the only independent predictor of subsequent isolation of a TR strain.T igecycline, a minocycline derivative, remains one of the few therapeutic options for the treatment of infections caused by multidrug-resistant Klebsiella pneumoniae (MDRKP) isolatesincluding Klebsiella pneumoniae carbapenemase (KPC) producers-and other Gram-negative organisms (1, 2). Of concern, tigecycline nonsusceptibility (i.e., tigecycline resistance [TR]) among Klebsiella pneumoniae isolates has been reported from different continents and ranged between 0% and 11.5% in a recent large study (3-6). Furthermore, several recent case studies identified patients who were treated with tigecycline for an initially tigecycline-susceptible MDRKP (TS-MDRKP) infection but from whom a TR-MDRKP strain was subsequently isolated (7-10). However, the potential of in vivo emergence of TR among MDRKP isolates has so far not been systematically investigated to our knowledge. We intended to study the rates of and risk factors for the in vivo emergence of TR among Klebsiella pneumoniae isolates in a large population of patients who initially presented with a TS-MDRKP.We performed a nested case-control study on the subsequent emergence of TR among all patients from whom a TS-MDRKP strain was previously isolated at a tertiary care center between January 2008 and July 2011. MDRKP was defined as the presence of either an extended-spectrum ␤-lactamase (ESBL)-or KPCproducing Klebsiella pneumoniae isolate, determined as per Clinical and Laboratory Standards Institute (CLSI) guidelines on ESBL testing, carbapenem resistance patterns, and Hodge test results, respectively (11, 12). Food and Drug Administration (FDA) breakpoints were applied to interpret the results of tigecycline susceptibility testing using the disc diffusion method and Etest (susceptible if the zone diameter is Ն19 mm and the isolate MIC is Յ2 g/ml, respectively).A case was defined as a patient from whom a TR-MDRKP was isolated between Ͼ48 h and Ͻ90 days after the day a TS-MDRKP strain was isolated (zero time). A control was defined as a patient from whom a TS-MDRKP strain was isolated during the study period but with no subsequent TR-MDRKP strains. Controls were matched to cases based on the isolate's resistance mechanism (ESBL versus KPC) but otherwise randomly selected in a 1:4 ratio. Cases were compared to controls with regard to a variety of demographic and clinical characteristics. We also recorded (i) all antibiotic exposures within 90 days before zero time and (ii) all antib...