In cystic fibrosis (CF) airway epithelial sodium absorption is increased 2-3 fold. Since sodium absorption is inhibited by the sodium channel blocker amiloride, our aim was to assess its therapeutic benefit in cystic fibrosis. A randomized, double-blind, placebo-controlled, cross-over trial of nebulized amiloride was performed in 23 patients with cystic fibrosis. Amiloride or placebo was administered four times daily for two six month periods. Existing treatment was continued, and any infective exacerbations treated in the usual way. Fourteen patients completed the study. No significant changes occurred in forced expiratory volume in one second, forced vital capacity, oxygen saturation, body weight, sputum volume, culture and rheology, serum urea, and electrolytes, white cell count and erythrocyte sedimentation rate during either treatment period. The frequency of infective exacerbations was also not different in either treatment period. We were thus unable to confirm the benefit shown in the only other clinical trial of nebulized amiloride in cystic fibrosis and conclude that, in the presence of established treatment for cystic fibrosis lung disease, nebulized amiloride offers no additional clinical benefit.