Nasal continuous positive airway pressure (nCPAP) is now the treatment of choice for patients with sleep apnoea syndrome. Side-effects and adverse reactions have been described in isolated reports with this device. We have, therefore, systematically studied the side-effects of nCPAP therapy in 300 consecutive patients referred to the London Chest Hospital Ventilatory Support Unit. Ninety-six per cent of patients complained of at least one side-effect resulting from the therapy, with 45 per cent complaining of a side-effect specific to the nasal mask. There was no correlation between the side-effects and level of pressure used during nCPAP. The rate of compliance remained high in spite of the side-effects, with a daily use of 7.8 hours (SD 8.05) and with 83 per cent of the patients using the device every night. Although nCPAP treatment remains acceptable to most patients there exists a high incidence of adverse effects.