Context: Studies in animals suggest that the limbic prefrontal cortex, including the anterior cingulate cortex, is involved in regulation of the hypothalamic-pituitary-adrenal (HPA) axis, but human data are lacking. Objective: This study tested the hypothesis that smaller anterior cingulate cortex volumes are associated with HPA axis dysregulation in healthy older men. Design and Participants: Comparison was made of volumes of bilateral anterior cingulate cortex, hippocampus, and superior frontal gyrus ( control region) volumes in two groups of 10 healthy men, aged 65 – 70 yr, who showed nonsuppression or suppression of cortisol levels in response to low dose ( 250 mu g) dexamethasone. Analysis of brain volumes was performed blind to the cortisol levels. Setting: This study was performed at a tertiary care clinical research center. Results: Nonsuppressors had significantly smaller left anterior cingulate cortex volumes than suppressors ( 5757 vs. 7817 mm(3); P = 0.01). Right anterior cingulate cortex, bilateral hippocampus, and bilateral superior frontal gyrus volumes were not significantly different between nonsuppressors and suppressors. Conclusions: Smaller left anterior cingulate cortex volumes may be associated with HPA axis dysregulation in humans. These results substantiate evidence from animal studies indicating an important role for the anterior cingulate cortex in suprahypothalamic feedback regulation of the HPA axis. The results also have implications for disorders in which HPA axis dysregulation and abnormalities of the anterior cingulate cortex are frequently observed, such as depression and Alzheimer’s disease.