Background: The object of this study was to investigate the antimicrobial resistance pattern among common nosocomial Gram-negative bacilli isolated from patients with nosocomial infections. Methods: From June 2004 to December 2005, 380 isolates of common Gram-negative bacilli (Klebsiella, Pseudomonas, Acinetobacter and E. coli) from 270 patients with nosocomial infections in Sina and Imam Hospitals, Tehran, Iran, were evaluated for susceptibility to Imipenem, Cefepime, Ciprofloxacine, Ceftriaxone and Ceftazidime by Disc diffusion and E-test methods. Results: The most frequent pathogens isolated were Klebsiella spp. (40%), followed by Pseudomonas (28%), Acinetobacter spp. (20%) and E. coli (12%). The most active antibiotic was imipenem (84%). 26% of all isolates were sensitive to Cefepime, 26% to Ciprofloxacin, 20% to Ceftazidime and 10% to Ceftrixone. The susceptibility rates of Klebsiella to Imipenem, cefepime, ciprofloxacin, Ceftazidime and Ceftriaxone were 91, 25, 21, 13 and 7 percent, respectively and 91, 19, 17, 21 and 21 percent, respectively, for E. coli. Among Acineto- bacter spp., the susceptibility rate was 77% for Imipenem and 21% for Ciprofloxacin. Among Pseudomonas spp., 75% of isolates were susceptible to Imipenem and 39% to Ciprofloxacin. The comparison of the resistance status of microorganisms by both Disc diffusion and E-test methods showed a clinically noticeable agreement between these two tests. Conclusions: Since antibiotic resistance among Gram-negative bacilli has increased, enforcement of policy regarding proper antibiotic use is urgently needed in order to delay the development of resistance. Although it is widely accepted that E-test is more accurate in determining the resistance of microorganisms, our study showed that the Disc diffusion test will give the same results in most occasions and is therefore still considered useful in clinical practice.