Gamma glutamyl transferase (GGT) is a new marker for predicting myocardial infarction, stroke, cardiac death and inflammation. There is also a strong relationship between Obstructive Sleep Apnea Syndrome (OSAS) and cardiovascular disease. This study was designed to investigate the association between serum GGT levels and cardiovascular disease in patients with OSAS, and relationship between severity of OSAS and serum GGT level. We evaluated the medical records of 166 subjects who were admitted for sleep study. OSAS was diagnosed by polysomnography if Apnea-Hypopnea Index (AHI) > 5. According to AHI, individuals in whom AHI < 5 were recruited as group 1 (OSAS negative group), AHI = 5-15: group 2 (mild OSAS group), AHI = 15-30: group 3 (moderate OSAS group), AHI > 30: group 4 (severe OSAS group). Cardiovascular disease was defined if the patients had heart failure, coronary artery disease or arrhythmia. Of the subjects, 112 (67.5%) were male and the mean age was 54.3 +/- 12.2 years. There were 22 patients (13.2%), 17 patients (10.2%), 34 patients (20.4%) and 93 patients (56.2%) in group 1, 2, 3 and 4, respectively. There is a significant increase in serum GGT levels while AHI score increases (group 1 = 28.0 +/- 10.1, group 2 = 33.8 +/- 13.2, group 3 = 35.2 +/- 8.5, group 4 = 40.0 +/- 22.0; p for trend = 0.024). However, serum C-reactive protein (CRP), alanine aminotransferase and aspartate aminotransferase levels were similar in all groups (p > 0.05). There was a significant independent association between serum GGT levels and the severity of OSAS. Moreover, serum GGT levels were significantly high in patients with cardiovascular disease compared with patients without cardiovascular disease in severe-moderate-mild OSAS (p < 0.05) and OSAS negative groups while CRP levels were not. This was a significant independent association. The present study suggests that high serum GGT level, regardless of the other traditional risk factors, is an independent predictor of cardiovascular disease in patients with OSAS. The results should be confirmed with other randomized prospective studies. (c) 2010 Elsevier Ltd. All rights reserved.