The first communications concerning stereotactic bilateral anterior capsulotomies were reported by Talairach and Leksell. This procedure has become established for the management of otherwise intractable anxiety neuroses and obsessive compulsive disorders, with a reported success rate of 70% in different series. It has been stressed that results are closely related to the extent of the lesion. The desirable lesion has a tubular shape with a length of 15-18 mm in the coronal axis. This shape is achieved by step withdrawal of a 4- to 5-mm electrode tip along a proper trajectory. A precise angulation of the trajectory is crucial to proceed without lesioning of the adjacent bordering caudate nucleus or putamen in the coronal plane. In the sagittal plane, it has to remain within the limits of the anterior capsula interna and avoid an excessive posteroanterior obliquity to ensure that the entry point through the cortex remains in the prefrontal noneloquent area. To achieve this trajectory, a target 5 mm posterior to the anterior border of the frontal horn, as seen on CT, at the level of the foramen of Monro has been suggested, along with a precoronal burr hole placed 20 degrees from the midline. Following these guidelines we have found the resulting lesions to be excessively anterior, with exclusion of their first 4- to 5-mm deep component. If the same target point is selected at two different axial levels to fix a trajectory, it usually results in an anteriorward trajectory that might require an entry point too close to the motor cortex, because the anterior horn reaches more rostrally as it deepens.(ABSTRACT TRUNCATED AT 250 WORDS)