AIM: To investigate whether children with cystic fibrosis under 3 years of age have disordered breathing and episodes of oxygen desaturation during sleep. METHODS: We studied 19 infants (9 boys and 10 girls) with cystic fibrosis, mean age 13.1 months (range 3-36 months) and 20 age and sex matched healthy subjects. Patients and controls underwent an overnight polysomnographic study and respiratory function testing on the following morning. RESULTS: Seven patients with ongoing respiratory tract inflammation had disordered breathing and episodes of oxygen desaturation during sleep. Pulse oximetry showed a significantly lower mean oxygen saturation (SaO(2)) and a higher percentage of total sleep time spent with SaO(2) less than 93% in symptomatic children than in controls. CONCLUSION: Results suggest that infants and young children with cystic fibrosis and mild airways inflammation (rhinitis, cough, red throat) have episodes of oxygen desaturation during sleep.