Abstract The use of combination antibiotic therapy for severe pseudomonal infections is a standard practice in many hospitals; however, the data supporting its use are somewhat unclear. Possible benefits of combination therapy for Pseudomonas aeruginosa infections include in vitro antibiotic synergy, prevention of the emergence of bacterial resistance while receiving therapy, and improved adequacy of empiric therapy. Unfortunately, the potential disadvantages are also considerable, the most worrisome of which are drug toxicity and creation of multidrug-resistant organisms in the environment. Many in vitro and animal studies have attempted to shed light on this clinically challenging issue; however, these studies have often yielded conflicting results and used different study methods, which limits the clinical utility of the results. Clinical studies have also attempted to clarify this issue, particularly in patients with serious pseudomonal infections such as bacteremia and ventilator-associated pneumonia, but again, often resulted in conflicting conclusions. Thus, we performed a MEDLINE search (1950-May 2010) of clinical and in vitro studies evaluating the use of antibiotic combination therapy and monotherapy for bacteremia and pneumonia due to P. aeruginosa. Although a clear answer still eludes this controversy, combination therapy for seriously ill patients suspected of having pseudomonal infection has been shown, with considerable evidence, to improve the likelihood of an active agent being included in the initial antibiotic regimen of these patients. The clinical status of the patient and true likelihood of encountering a multidrug-resistant organism should be evaluated before deciding on empiric combination therapy. Future research may be able to better identify which patient populations might receive the most benefit from combination therapy rather than using combination therapy for everyone at risk for these infections.