Pulmonary exacerbations of cystic fibrosis associated with strains of Pseudomonas aeruginosa that are resistant to multiple antibiotics are becoming increasingly common. The search for treatment alternatives continues and may include the reexamination of older antibiotics. Colistin sulfate is a polypeptide antibiotic with good activity against P. aeruginosa. Although its use was largely discontinued in the early 1970s because of reports of frequent renal and neurologic toxicity, intravenous colistin is often prescribed at our institution for patients with P. aeruginosa resistant to multiple-drug therapy. We prospectively monitored 19 patients during 21 courses of colistin therapy to identify the character and incidence of this agent's toxicity. Only one case of renal toxicity occurred. Six cases of neurotoxicity occurred, which were characterized by perioral paresthesia, ataxia, or both. The rate of intolerable renal adverse effects secondary to colistin therapy was appreciably lower among these patients than that reported previously for other patients. It appears that intravenous colistin can be considered for cystic fibrosis patients with strains of P. aeruginosa that are resistant to more commonly used antibiotics.