Twenty patients (17-27 yr) with cystic fibrosis were given ciprofloxacin at 30 pulmonary infectious exacerbations. All patients were chronically colonized with Pseudomonas aeruginosa. Twenty-five courses were completed, 13 orally (15 mg/kg b.i.d.) and 12 intravenously (4-6 mg/kg b.i.d.). Clinical efficacy was excellent or good in 85-90% of the courses and growth of P. aeruginosa was markedly reduced in 33-46%. Body weight and clinical score improved significantly. White blood cell count decreased and pulmonary function was improved. Reversible adverse effects, mainly rash and urticaria, appeared at seven occasions, five severe enough to cause interruption of treatment. Clinical efficacy and tolerance were better with oral than intravenous administration at the dosages used in this study. Excellent bioavailability provides additional basis for oral treatment with ciprofloxacin in cystic fibrosis patients.