The aim of this study was to determine wether clinical, spirometric, or arterial blood gas data could predict sleep apnea syndrome (SAS) among obese patients with at least one functional complaint. Eighty three obese patients, 46 males and 37 females, aged 53.2 +/- 13.7 years, with a body mass index (BMI) above 30 kg/m2 were investigated with: clinical questionnaire; pulmonary function tests; arterial blood gas measurements; nocturnal monitoring of oronasal airflow, thoracic and abdominal movements, and arterial oxygen saturation. Thirty five patients (42.2%) had a SAS, 18 (26.7%) had nocturnal oxygen desaturations, and 30 (36.1%) had a normal nocturnal monitoring. There were 63% males in the SAS group and 32% in the group without SAS (P < 0.01). Age, weight, frequency of systemic hypertension, frequency of each clinical symptom, spirometric and arterial blood gas data were identical in the two groups. In conclusion, rate of SAS is very high in obese patients presenting a BMI above 30 and at least one functional complaint. No predictive test is identified.