The objective of this study was to determine whether different decreases in oxygen saturation (SaO2) or the presence of electroencephalographic arousals (EEGA) in the definition of hypopnea modify hypopnea index and apnea/hypopnea index and the prevalence of obstructive sleep apnea/hypopnea syndrome (OSAHS). A total of 20 polysomnographies performed in patients with OSAHS were analyzed. There are four different definitions of hypopnea: > or = 30% reduction in airflow or 50% decrease in abdominal movement associated with decreases SaO2 > or = 3% (type 1); decreases SaO2 > or = 3% or EEGA (type 2); decreases SaO2 > or = 4% (type 3); decreases SaO2 > or = 4% or EEGA (type 4). The prevalence of OSAHS was calculated for an apnea/hypopnea index (AHI) > or = 10 and > or = 15. Hypopnea index (HI) and AHI types 2 and 4 were higher than type 3 (HI: type 2: 20 +/- 10.6, type 4: 18.6 +/- 10, type 3: 11.4 +/- 10, p 0.05). The prevalence of OSAHS was 30-55% in type 3, 70-85% in type 4 (p 0.05). In our patient's population, the presence of EEGA in the definition of hypopnea significantly increased the HI, the AHI and the prevalence of OSAHS when associated with a > or = 4% decrease in SaO2.