Obstructive sleep apnea syndrome (OSAS) is characterized with interruption of sleep with apnea attacks and is associated with various diseases such as metabolic syndrome. GDF-15 (Growth differentiation factor 15) is a cytokine belonging to the transforming growth factor – β superfamily and it has been found to be associated with chronic heart failure, acute pulmonary embolism, and acute attacks of chronic obstructive pulmonary disease (COPD). It was aimed to evaluate the relationship between disease severity and mean GDF-15 in patients diagnosed with OSAS. Our study was carried out with 75 patients with OSAS. GDF-15 levels were checked by in the morning fasting blood test. Among the 75 patients, 27 constituted the mild, 22 constituted the moderate and 26 constituted the severe OSAS groups. The mean ages of the patients were not significantly different between groups (53.17±10, 54.61±10.2 and 53.5±11.6 respectively; p>0,05). GDF-15 levels were not significantly different between groups separately (p>0.05). There was only a weak positive relationship between GDF-15 and NREM and REM (p<0.05). In our study, it was revealed that there was no significant relationship between the OSAS severity groups and GDF-15. Our determination of a positive relationship with NREM and REM may have been related to reduction of minute ventilation experienced in OSAS-diagnosed patients, tachycardic fluctuations, their more severe nature and increased right ventricular pressure. Consequently, our current knowledge indicates that GDF-15 is not much guiding in the prediction and monitoring of OSAS severity.