52 children with severe cough persisting for more than 10 days were randomized to treatment with amoxycillin/clavulanic acid or placebo in a prospective double-blinded study. Clinically suspected cases of pertussis were excluded, yet 12 (23%) of the children had laboratory verified pertussis infection. The nasopharyngeal colonization showed a predominance of Moraxella catarrhalis which was isolated in 37 (71%) children. Streptococcus pneumoniae and Haemophilus influenzae were isolated in 11 (20%) and 16 (30%) children, respectively. The antibiotic-treated group had a significantly better recovery in both the pediatrician's estimation (p = 0.02) and the independent parental judgement (p = 0.002). These findings are consistent with the view that Moraxella catarrhalis could be directly involved in the pathogenesis of persistent cough in children.