Obstructive sleep apnea/hypopnea syndrome (OSAHS) is an important and more common public health problem with increasing incidence. Polysomonography (PSG) is the gold standard test in OSAHS diagnosis. Apnea-hypopnea index (AHI) is the main parameter of PSG, which is correlated with OSAHS severity. The main complaint of OSAHS patients is daytime sleepiness and the Epworth Sleepiness Scale (ESS) used for evaluation of disease severity. The correlation of AHI with daytime sleepiness and ESS is well known. But there are many patients, which have uncorrelated daytime sleepiness with AHI. This data calls this hypothesis; Are there any other parameters which may affect daytime sleepiness. 648 patients with complaining of snoring and apnea were evaluated by polysomnography and anthropometric measurements. The cut-off value of ESS was accepted 10 as an indicator of severe daytime sleepiness. Patients were divided to groups with the aim of homogenization, according to AHI values. The patients with similar AHI values were analyzed according to their ESS scores. BMI and neck circumference were elevated in daytime sleepiness patients. The nocturnal hypoxemia markers; apnea number/index, maximum duration of apnea, at least SO2 concentration, duration of SO2 less than 90 % were much effected in the group of daytime sleepiness. Beside the fact that our research, AHI is not enough for predicting the daytime sleepiness; anthropometric measurements and the nocturnal hypoxemia markers should be evaluated.