THE AUTHORS RETROSPECTIVELY reviewed 117 pediatric patients with posterior fossa tumors treated at The Children's Hospital and Medical Center, Seattle, Washington, between 1976 and 1990, in an attempt to determine what perioperative and intraoperative factors influenced the need for postoperative shunts. The ages of the patient population ranged from 4 months to 16 years 9 months. The factors evaluated included age at diagnosis, duration of symptoms, extent of hydrocephalus, tumor location, extent of tumor resection, presence and duration of an external ventricular drain, flow of cerebrospinal fluid (CSF) through the fourth ventricle after tumor resection, presence of hemostatic cavity linings, method of dural closure, tumor type, CSF infection, CSF leak, and pseudomeningocele formation. Of these variables, young age at diagnosis, tumors affecting midline structures, subtotal tumor resection as determined by immediate postoperative scans, prolonged requirement of an external ventricular drain, cadaveric dural grafts, pseudomeningocele formation, and CSF infections were statistically significant factors associated with the need for postoperative shunt placement, which was required in 36% of all patients. Therefore, considering the pertinent factors that may influence the need for postoperative shunt placement, a radical tumor resection should be done when possible, especially for midline lesions, and closure of the operative wound should be meticulous to avoid a CSF leak and subsequent infection. Foreign body dural substitutes as a cause of an inflammatory response in the CSF should also be avoided. Persistence of a pseudomeningocele despite serial taps and time will eventually require placement of a shunt after posterior fossa tumor surgery.