The authors report on a neonatal patient with traumatic subacute subdural effusion in the posterior fossa associated with secondary acute hydrocephalus. The infant fell from his mother's hand onto the floor, injuring his left parietal region. Computed tomography (CT) scans of the patient's head revealed a linear fracture of the left parietal bone, a small contusion in the right temporal lobe, and a small subdural hematoma in the right posterior fossa with thin subdural effusion. Serial CT scans revealed a progressive increase in subdural effusion bilaterally in the posterior fossa. On Day 7 the anterior fontanelle was tense and CT scans revealed marked hydrocephalus associated with thick subdural effusion in the posterior fossa. External drainage of both the subdural effusion and dilated lateral ventricles improved the patient's condition, and no reaccumulation of subdural effusion has been observed. The origin and treatment of this rare clinical entity is discussed.