Dorsal midbrain syndrome (DMS) is a recognized clinical manifestation of increased intracranial pressure (ICP) associated with ventricular enlargement, especially in shunt malfunction, but the mechanism by which DMS occurs in this setting is unsettled. We report a patient with triventriculomegaly attributed to aqueductal narrowing by a tectal mass who went through 2 cycles of developing and resolving DMS promptly after undergoing interventions that altered the size of the posterior third ventricle and proximal aqueduct but probably did not markedly alter ICP. This case provides additional evidence that DMS in this setting is caused by deformation of the dorsal midbrain region produced by rapid expansion of the posterior third ventricle or proximal aqueduct.