The clinical efficacy of the conventional aminoglycoside plus beta-lactam treatment was compared to that of monotherapy with oral quinolones in 26 adult cystic fibrosis patients in an open prospective clinical trial in which six two-week courses of antipseudomonas treatment were administered with an interval of approximately three months between treatments. In each patient two courses of conventional treatment were followed by two courses of quinolone treatment and then by another two courses of conventional treatment. The observed improvements in pulmonary function were somewhat higher when the patients received conventional treatments, and in the most seriously affected patients conventional treatment was significantly better than quinolone treatment. On the basis of these findings it is suggested that quinolone monotherapy cannot replace conventional antipseudomonal chemotherapy in patients with severe pulmonary involvement.