The isolation of Nocardia species from the respiratory secretions of patients with cystic fibrosis presents problems with important clinical implications. From the sputum culture of a total of 387 patients with cystic fibrosis, Nocardia species was isolated in 9 cases (2%; 8 females and 1 male) with a mean (SD) age of 17 (7) years. Sixty-seven percent of the patients were asymptomatic and no relevant radiographic or analytical changes were detected. In only 3 patients was of Nocardia species isolated again in successive samples. Two patients were not treated, 7 were treated with cotrimoxazole and 3 with minocycline; in 2 cases therapy was intravenous. After a mean follow-up of 48 (33) months, all patients had improved. Isolation of Nocardia species from the secretions of patients with cystic fibrosis does not necessarily imply infection and the need for treatment should be assessed on an individual basis.