During the period 1977-81, 188 Cystic fibrosis patients have been treated with 1 254 anti-haemophilus treatments in the Danish Cystic Fibrosis Center. On an average, each patient received 1.7 courses of chemotherapy per year. Haemophilus infections were more prevalent in the youngest age group of patients. The average number of H. influenzae precipitins increased with age, and this pattern was consistent throughout the whole study. The standard treatment was orally administered pivampicillin or amoxicillin given for 14 days. 1 case of allergy or ampicillin-resistant strains, erythromycin was given, in most cases alone, in some cases in combination with rifampicin. The overall results showed, that H. influenzae was eradicated from sputum by a single course of chemotherapy in 73% of the courses and H. parainfluenzae in 100% of the courses. Six per cent of the patients suffered from chronic H. influenzae infection in spite of repeated treatments. No decrease of treatment efficiency was found even after many repeated chemotherapy courses. The prevalence of patients harbouring ampicillin-resistant H. influenzae increased in the beginning of the period (6%-20%), but a slight fall was subsequently observed.