Nearly all strains of Pseudomonas aeruginosa are sensitive to colomycin sulphomethate, but studies in the 1970s using large doses demonstrated significant renal and neurotoxic side-effects and it is not now commonly used. In this study colomycin (2 megaunits i.v. t.d.s.) has been used extensively in adult cystic fibrosis (CF) patients and its use reviewed to determine its efficacy and safety profile. Fifty-two CF patients (28 male, 24 female; mean age 26 yrs, range 17-39 yrs) received 135 courses (mean two courses each, range 1-7, median length 14 days) of i.v. colomycin (2,414 patient days in total). It was used in combination with one other i.v. antibiotic in 114 courses (85%) and with two others in 18 (13%). In all cases there was significant improvement in spirometry (pretreatment forced expiratory volume in one second (FEV1) % predicted mean 44.4, range 10-101; post-treatment mean 51.3, range 14-108; p<0.0001). No patient had any neurotoxicity but one developed a skin rash and myositis. There was no change in renal function (urea mean pretreatment 4.1 mmol x L(-1) (sD 1.4), mean post-treatment 43 (2.2), p=NS; creatinine mean pretreatment 77.9 mmol x L(-1) (15.3), mean post-treatment 803 (21.6), p=NS). In the authors' experience intravenous colomycin sulphomethate in moderate doses is an effective and safe antipseudomonal antibiotic which is easy to administer. Other clinicians should consider its use in patients with cystic fibrosis.