Established respiratory infections with mucoid Pseudomonas aeruginosa in patients suffering from cystic fibrosis were treated with conventional as well as larger doses of tobramycin. The infection was eradicated in four of the 17 patients treated, but the duration of follow-up study of one patient was short. It appeared that treatment was most successful in those patients in whom the highest peak concentrations of tobramycin in sputum were obtained. No side effects were noted, even when tobramycin was given in large doses for two weeks.