18 children with cystic fibrosis and mucoid pseudomonas lung infection were treated with courses either of gentamicin plus carbenicillin, or tobramycin plus carbenicillin, with 2 children each receiving two courses. 10 courses of gentamicin at a dose of 9 mg/kg per day plus carbenicillin at 800 mg/kg per day, and 10 courses of tobramycin at 9 mg/kg per day plus carbenicillin at 800 mg/kg per day were given. There was clinical and x-ray improvement in both groups of children, but there was no difference between the therapeutic benefit of either regimen. Pseudomonas aeruginosa was not cultured at the end of treatment after 15 of the 20 courses, but it returned in all but one patient within 3 months. Neither ototoxicity nor renal damage with these high doses of aminoglycoside was detected. P. aeruginosa had not been eliminated when 9 of these patients earlier had received courses of gentamicin in a dose of 6 mg/kg per day plus carbenicillin at 800 mg/kg per day. The results show that P. aeruginosa can successfully be eliminated or suppressed with high-dose aminoglycoside plus carbenicillin, but such elimination is usually short lived.