Information on the function of the hypothalamic-pituitary-adrenal (HPA) axis, the main mammalian system of stress response, in obsessive compulsive disorder (OCD) is inconsistent. In this study, nine inpatients with a DSM-IV diagnosis of OCD without comorbid major depression (Yale-Brown Obsessive Compulsive Scale [Y-BOCS] score >15; HAMD-21 total score 16) and nine healthy matched controls were included. Blood of patients (seven males; 31.8 +/- 9.3 years, Y-BOCS: 27.3 +/- 4.3, HAMD-21: 13.3 +/-1.9) and controls (seven males, 31.6 +/- 9.1 years) was drawn every 20 min between 23:00 and 7:00 h during sleep using a long catheter for later ACTH and cortisol analysis. Secretion patterns of cortisol and ACTH were similar in both groups, in OCD, however, at a higher level. Area under the curve plasma concentrations of both ACTH (p<0.05) and cortisol (p<0.005) were significantly greater in patients with OCD (ACTH: 674.3 +/- 57.4; cortisol: 2148.4 +/-271.7) than in controls (ACTH: 460.2 +/- 61.0; cortisol: 1191.2 +/- 124.1). In conclusion, our findings suggest that the activity of the HPA axis in patients with OCD is increased compared to healthy controls.