The objective of this study was to describe the incidence and types of visual field defects after posterior globus pallidus internus (GPi) pallidotomy for Parkinson's disease. The creation of the pallidotomy lesion carries a risk of damaging neighboring structures such as the optic tract. The reported frequency of visual field defects in patients after pallidotomy varies from 0 to 40%. Goldmann visual field testing was performed on 40 patients who underwent microelectrode-guided posterior GPi pallidotomy. The optic tract was identified during the procedure by listening during microelectrode recording for the evoked responses to light flashes and by assessing stimulation-induced subjective responses. After the first 18 patients, lesioning thresholds were increased from 0.5 to > or =1.0 mA so that the lesion was placed more distant from the optic tract. The location of individual lesions was determined on postsurgical MRI. Three patients (7.5%) had visual field defects likely related to the pallidotomy. These were contralateral homonymous superior quadrantanopias, associated in two patients with small paracentral scotomas. The incidence of visual field defects with the early technique was 11% (2/18) and decreased to 4.5% (1/22) after thresholds for lesioning were increased. Except for the location of the lesion relative to the optic tract (more ventral, adjacent to or extending into the optic tract), no other variable correlated with a post-pallidotomy visual field defect. Microelectrode-guided GPi pallidotomy is a relatively safe procedure as regards visual function even when the optic tract is used as a guide for lesion placement.