Nosocomial infections by resistant gram-negative microorganisms are important causes of mortality in intensive care unit (ICU)'s. The treatment choices are limited in infections due to Acinetobacter baumannii and Pseudomonas aeruginosa, especially if they are panresistant. In these type of resistant infections, colistin--an old antibiotic--has become a current issue. The aim of this study was to evaluate the efficacy of colistin in 9 cases (6 males, mean age 75.8 +/- 9.4 years), with ventilator associated pneumonia (VAP) caused by panresistant A. baumannii and P. aeruginosa in respiratory ICU. All cases were referred to ICU from other hospitals or clinics. It was detected that 7 of 9 cases were treated with anti-pseudomonal antibiotics before the development of VAP. Panresistant A. baumannii was isolated in 5 cases and P. aeruginosa in 4 cases. VAP by these microorganisms was detected on the 26.6 +/- 12.4th days of invasive mechanical ventilation and the cases were followed up for 54.2 +/- 25.7 days in ICU. During colistin treatment, dermatitis (one case) and nephrotoxicity (one case) were observed as side effects. Microbiological response to colistin was obtained in 6 cases. Three cases died due to non-eradication of panresistant microorganisms and three cases died due to other infections during ICU follow-up. The data presented in this study demonstrates that colistin can be considered as a safe and effective antibiotic in the treatment of panresistant A. baumannii and P. aeruginosa infections.