Non-dipper hypertension is associated with increased cardiovascular morbidity and mortality. We aimed to evaluate serum gamma-glutamyl transferase (GGT) level, which is accepted as a marker for oxidative stress and its relationship with inflammatory activity in patients with non-dipper hypertension. Age and sex matched 43 dipper hypertensive patients, 40 non-dipper patients, and 46 healthy subjects were included into the study. Serum GGT and C-reactive protein (CRP) levels were measured and compared between each of the groups. Serum GGT activity was higher in the non-dipper and the dipper hypertensive groups than in the control group (33.5 +/- 11.8 and 28.1 +/- 10.1 U/l, respectively, vs. 21.2 +/- 6.5U/l; p < 0.001). There was a statistically significant difference in serum GGT activity between the non-dippers and the dippers (p = 0.021). When compared with the control group, serum CRP levels were significantly increased in both the non-dipper and the dipper hypertensive groups (6.1 +/- 2.6 and 5.4 +/- 2.1 mg/l, respectively, vs. 2.8 +/- 1.7mg/L; p < 0.001). Increased CRP levels were higher in non-dippers than dippers (p = 0.046). A significant correlation was found between GGT and CRP measurements (r = 0.37, p = 0.002). Serum GGT levels, which are markers of the oxidative stress and CRP levels, are both increased in non-dipper hypertension. Increased GGT activity, found to be correlated with CRP levels, may be one of the reasons behind the non-dipper hypertension related cardiovascular complications.