We present nine cases of obstructive hydrocephalus (OH) associated with marked parkinsonism. Four patients had noncommunicating OH (NCOH) [three nontumoral aqueductal stenosis (AS), one tumoral AS]. The presentation was that of acute or subacute parkinsonism, usually at the time of acute recurrent ventricular obstruction. Three had a marked response to levodopa and required short-term treatment after shunting. However, one has remained levodopa dependent after 2 1/2 years. Three of the five patients with communicating OH (COH) presented with shunt-responsive normal pressure hydrocephalus (NPH), only later to develop progressive parkinsonism. One of these was found to have progressive supranuclear palsy (PSP) at autopsy and PSP was clinically suspected in one other patient. A third had an atypical course suggestive of PSP; however, autopsy demonstrated the combination of Lewy body parkinsonism and the sequelae of hydrocephalus. The remaining two COH patients presented with levodopa-responsive parkinsonism. Subsequent clinical features and imaging studies suggested the presence of NPH. The pathophysiology of hydrocephalic parkinsonism probably involves variable sites of dysfunction in the nigrostriatal pathway and/or the cortico-striato-pallido-thalamo-cortical circuit. At certain locations these pathways lie in close proximity to the ventricular system and may be subjected to mass effects and ischemic changes secondary to ventriculomegaly. The additional importance of possible associations between subcortical cerebral ischemia, NPH, and "degenerative" disorders such as PSP and Parkinson's disease is discussed.