Nasal continuous positive airway pressure (nCPAP) is the current treatment of obstructive sleep apnoea syndrome (OSAS). The indications of bilevel pressure support ventilation (BIPAP PSV) in OSAS patients remain controversial. The purpose of this investigation was to verify the frequency of prescription of BIPAP PSV in a group of OSAS patients when CPAP was ineffective or not tolerated during titration. The study included 286 consecutive patients > or = 18 years of age referred to two Sleep laboratories for sleep related breathing disorders (SRBD) between December 1994 and November 1995. Of these, 130 patients were enrolled and 105 (88 males, 77 females) with moderate to severe OSAS completed the study and were finally analysed. After a full night diagnostic polysomnography (PSGD), patients had a second full night PSG under nCPAP (PSGT). If nCPAP was not tolerated, or failed to correct breathing abnormalities during sleep, a second PSGT was performed, using a BIPAP PSV. Our study shows that nCPAP (mean 8.5 +/- 2.0 cmH20) was considered a satisfactory therapy in 81 patients (77%). Twenty four (23%) required BIPAP PSV (mean IPAP 13.9 +/- 2.9 cmH20). We found the highest prevalence of BIPAP in patients with OSAS associated to obesity hypoventilation syndrome (OHS) (11 of 17) and in OSAS associated to chronic obstructive pulmonary disease (COPD) (nine of 16). Patients treated with BIPAP PSV were more obese and had a higher PaCO2 and sleep-related desaturations and a lower FEV1, FVC, FEV1/FVC and PaO2. In conclusion our study shows that CPAP therapy in the effective therapeutic option in the majority of patients with OSAS. There is a subset of patients with OSAS associated to COPD or to OHS in whom BIPAP PSV may be a better treatment modality.