The polymyxins (colistin and polymyxin B) are basic polypeptides that are bactericidal for certain gram-negative aerobic rods, including Pseudomonas. Because of poor distribution into tissues and substantial toxicity (primarily nephrotoxicity and neurotoxicity), systemic use of these agents has been limited to infections caused by multidrug-resistant gram-negative organisms that are sensitive only to the polymyxins. Colistin has been used with increasing frequency in the treatment of pan-resistant Acinetobacter baumanii and P aeruginosa. In the treatment of cancer patients with multidrug-resistant P aeruginosa, colistin has been found to be more effective than other antibacterial options. The more recent experience suggests colistin to be associated with less nephrotoxicity and neurotoxicity than previously described. Dosage adjustments are required with decreased renal function.