The aim of this study was to evaluate the efficacy of cognitive therapy (CT) in the treatment of generalized anxiety disorder (GAD), as it would be reflected through both psychological and psychoneuroendocrinological parameters. For this purpose, a group of outpatients with GAD were treated with CT for up to a maximum of 24 sessions. In order to assess psychological and biological changes, anxiety-related symptoms were evaluated according to the Hamilton Anxiety Rating Scale (HAM-A), and the hypothalamic-pituitary-adrenal (HPA) function was determined through assessment of circulating cortisol levels. A significant decrease in the HAM-A scores, along with significant changes in plasma cortisol levels, were observed after completion of treatment with CT. These observations contribute to demonstrate that the effect of certain psychotherapeutic approaches, such as CT, may be observed at both psychological and biological levels.