The benefits of antibiotic treatment and a nasopharyngeal culture in children with longstanding cough were analysed in a prospective randomized open study. Clinically suspected pertussis was excluded. Of 40 children given erythromycin for 7 days, 35 (88%) recovered in one week, compared with 17/47 (36%) untreated (p < 0.0001). Erythromycin eliminated Moraxella catarrhalis from the nasopharynx in 21/31 children (68%), compared with spontaneous disappearance in 7/35 (20%) untreated controls (p < 0.001). Purulent bronchitis or otitis media occurred in 2 children (5%) in the treatment group and in 21 (45%) in the control group (p < 0.01). To evaluate the clinical role of isolated pathogens, the 47 untreated subjects were studied. Seven of 35 children harbouring M. catarrhalis recovered, compared with 8/12 in whom this bacterium was absent (p < 0.01). No correlation was found between the isolation of Haemophilus influenzae or Streptococcus pneumoniae and the clinical outcome. Children with persistent cough > 10 days may benefit from erythromycin treatment. M. catarrhalis in the nasopharynx indicates prolonged symptoms and increased risk of bacterial complications.