Introduction: Early detection and treatment of sepsis can be lifesaving for critically ill patients. Sepsis caused by Gram negative organisms is on the rise in cancer patients. It is important to identify the infecting organism and test the effectiveness of antibiotics against it as soon as possible. We aimed at studying the in vitro effectiveness of commonly used antibiotic against pathogenic Gram negative organisms in cancer patients with sepsis. Methods: We conducted a four year and one month study of all Gram-negative isolates from blood samples of patients received from oncology units in our hospital. All isolates were processed as per standard microbiology laboratory operating procedures (SOPs). Isolates were identified to species level and susceptibility tests were performed, interpreted and reporting of results was as per Clinical Laboratory Standards Institute (CLSI) guidelines. Results: A total of 5391 blood cultures were received during the study period of which 179 tested positive for Gram negative bacterial sepsis. Escherichia coli Klebsiella pneumoniae, Pseudomonas aeruginosa, and Acinetobacter spp. were most commonly encountered pathogens. Resistance to piperacillin-tazobactam in the Enterobacteriaceae group was high at 63.46% in K. pneumoniae and 46.4% in E. coli respectively. Although resistance to cefoperazone–sulbactum was 65% in K. pneumoniae, three-fourths of the isolates remained susceptible to ceftazidime. K. pneumoniae and E. coli also showed a very high rate of resistance 69.3% and 83.2% respectively to the fluoroquinolone ciprofloxacin. Half of the Acinetobacter strains were resistant to meropenem a carbapenem antibiotic. Three isolates of Klebsiella pnuemoniae were resistant to Colistin. Conclusion: A high level of resistance to cephalosporins, fluoroquinolones and beta-lactam – beta lacatamase inhibitor combinations is seen in Gram negative sepsis causing organisms in cancer patients, particularly E. coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, and Acinetobacter spp. Resistance to carbapenems is also on the increase in this group of organisms.