Nine patients with cystic fibrosis have been treated with azlocillin alone and later with azlocillin combined with an aminoglycoside (gentamicin or tobramycin) for 50 treatment courses. In the initial series when azlocillin was employed alone, a gradual increase in MIC during successive courses was observed in Pseudomonas aeruginosa. When the beta-lactam antibiotic was combined with an aminoglycoside, the MIC was either maintained or reduced. Objective criteria like peak expiratory flow, erythrocyte sedimentation rate, fever, body weight or bacterial cultures could not clearly identify the combination therapy as better clinically than azlocillin monotherapy. However, the patients subjective and our clinical impression is that the combination therapy was better. The clinical course and the lack of increased resistance on combination therapy make a combination of azlocillin and an aminoglycoside preferable to the beta-lactam alone.