BACKGROUND: Anterior cingulotomy (ACING) is a neurosurgical treatment for chronic refractory depression, pain, and obsessive-compulsive disorder. Anterior cingulotomy involves the placement of bilateral lesions in the anterior cingulate under stereotactic guidance. Although a long-established therapeutic intervention, the optimal location and volume of lesions are not known, but it is generally believed that efficacious lesions interrupt the fibers of the cingulum bundle. METHODS: Using T2-weighted magnetic resonance imaging, we tested the hypothesis that lesions placed more anteriorly would be associated with a better clinical response. We also tested a secondary hypothesis that a superior clinical response would be associated with larger lesion volumes. RESULTS: When assessed 12 months following surgery, a superior clinical response was associated with more anterior lesions but, unexpectedly, with smaller lesion volumes. Specifically, the best clinical response was associated with total (right plus left hemisphere) lesion volumes of 1000 to 2000 mm(3) centered at Montreal Neurological Institute (MNI) coordinates (+/- 9,19,30). CONCLUSIONS: There is considerable evidence from neuroimaging studies that more rostral areas within the anterior cingulate cortex are functionally and structurally abnormal in patients with major depressive disorder. Anteriorly placed ACING lesions would target and modify function within such regions. It should not be assumed that larger lesions are associated with a better response. These findings of relationships between lesion characteristics and clinical response argue against the suggestion that ACING represents a placebo treatment.