Forty patients with Parkinson's disease without mental deterioration who were scheduled for ventroposterolateral (VPL) pallidotomy were randomly selected for retrospective stereotactic magnetic resonance image (MRI) analysis. The preoperative MRI study was performed on a 1.0-T MRI machine with a three-dimensional gradient-echo sequence. The MRI analysis was focused on five consecutive 2 mm thick axial slices without gap and parallel to the intercommissural line, starting from the level of the foramen of Monro and continuing in a ventral direction. Lacunar cysts of varying sizes (4-424 mm3) were seen at least in one hemisphere of all patients. The cysts had a clear dominance in posteroventral regions of the lateral-most pallidal regions (GP) and posteroventral regions of the putamen (PUT). No statistical correlation was found between the number or volume of the cysts and the sex, age, or duration of illness of the patients. Patients with predominantly left-sided clinical symptoms had a concentration of the cysts in the left GP, whereas those with predominantly right-sided symptoms had cysts significantly larger and more frequent in the right than the left GP. The cysts did not seem to affect the clinical outcome of pallidotomy. The authors think striatopallidal cysts develop from dilated perivascular spaces of the lenticulostriate vessels in the posteroventral regions of the GP and PUT. They are not pathognomonic for PD, but they may play some role in lateralization of the clinical symptoms in this classically asymmetric condition.