The role of the expiratory phase in obstructive sleep apnoea (OSA) is not well known. The aim of our study was to verify the contribution of expiratory narrowing to apnoea in a group of OSA patients by evaluating the effects of short-term treatment with continuous positive airway pressure (CPAP), intermittent positive pressure ventilation (IPPV) and bi-level positive airway pressure (BIPAP). We studied a selected group of 10 OSA patients whose therapeutic pressure level of CPAP was at least 10 cm H2O. During CPAP therapy the mean apnoea/hypopnoea index (AHI) and oxyhaemoglobin desaturation index (ODI) decreased from 64.8 to 6.3 (P < 0.001) and from 58.5 to 6.1 (P < 0.001), respectively and mean nadir SAO2 increased from 62.0 to 91.6 (P < 0.001). None of the patients reached optimal setting (elimination of snoring, reduction of apnoeas and non-apnoeic desaturation events at least to 15 or less per hour of sleep and maintenance of oxygen saturation approximately 90%) during IPPV and two patients did not tolerate final IPAP pressure levels. When a critical level of EPAP (BIPAP) was applied in the same night to these patients, optimal setting was reached in all subjects. During BIPAP, mean AHI decreased from 64.8 to 7.4 (P < 0.001); ODI decreased from 58.5 to 7.6 (P < 0.001) and nadir SAO2 increased from 62.0 to 91.2 (P < 0.001). Our study confirms the essential role of a critical level of EPAP in successful ventilatory treatment in OSA, thereby indicating, in agreement with few previous studies, the critical role of end of expiratory occlusion in OSA pathogenesis.