BACKGROUND: Derepressed AmpC beta-lactamase producing Enterobacter cloacae, Citrobacter f reundii, and Serratia marcescens are important nosocomial pathogens and the infections are difficult to treat, because they are multi-drug resistant. The aim of this study was to determine the isolation rate and trend, and antimicrobial susceptibility of derepressed strains isolated from clinical specimens. METHODS: E. cloacae, S. marcescens, and C. f reundii isolated from 1996 through 2000 were enrolled in the study. Antimicrobial susceptibility was tested by NCCLS disk diffusion method. Derepressed strain was defined as strain non-susceptible to third generation cephalosporin. The isolation patterns of important gram-negative bacilli with the derepressed strains were analyzed with respect to years, patient's locations and specimens. RESULTS: Among the clinical isolates, the derepressed strains of E. cloacae, S. marcescens, and C. f reundii were 65%, 70%, and 56%. The proportion of the derepressed strains : E. cloacae increased from 68% in 1996 to 71% in 1998, however, decreased to 59% in 2000, S. marcescens increased from 68% in 1996 to 73% in 2000, C. f reundii decreased from 69% in 1996 to 41% in 2000. The proportion of the derepressed strains were high among the isolates from blood and respiratory specimens of inpatient and intensive care patient. The resistance rates of the depressed strains were 47~62% to third generation cephalosporin and aztreonam, 15~85% to aminoglycoside, 68% to cotrimoxazole, and 31% to levofloxacin. CONCLUSION: Among the clinical isolates of E. cloacae, S. marcescens, and C. f reundii, the derepressed strains were as high as 56~70%, and they were commonly isolated from blood and sputum specimens of inpatient and intensive care patient, and showed high resistance rates to the most antimicrobial agents.