The morbidity and mortality of obstructive sleep apnea (OSA) are related principally to its cardiovascular complications. Metabolic syndrome (MBS) is recognized as raising the risk of cardiovascular disease. In this study, we analysied the diagnostic value of MBS to predict the diagnosis of severe OSA. Eight-seven subjects (54 males and 33 females) without any cardiac or pulmonary disease referred for evaluation of OSA, had overnight polysomnography. MBS were diagnosed according to NCEP criteria. According to apnea-hypopnea index (AHI), subjects were divided into two groups: severe OSA (AHI>or=30, 26 males, 15 females) and non-severe OSA (AHI<30, 28 males, 18 females). Ages were similar in both OSA groups for both genders. In the severe OSA group, number of patients with MBS was especially higher (in females n=13, 86.6%; in males n=19, 73%) than non-severe OSA (in females 6, 33%, in males 9, 32%). The mean values of the five diagnostic criteria of MBS were significantly higher in severe OSA than non-severe groups in both genders. Analysis of diagnostic value (criteria>or=3) of MBS for predicting the diagnosis of severe OSA in males and females revealed the sensitivity 73% and 86.6%, the specificity 67.8% and 94.4%, positive predictive value 67.9% and 86.7%, negative predictive value 73% and 85.7% respectively. Especially in female patients referred to the sleep clinics, evaluation of MBS criteria may play an important role in predicting severe OSA with high sensitivity and negative predictivity. Furthermore, the presence of MBS in OSA patients might increase cardiovascular complications.