The Simpson grading system classifies incomplete resections into a single category, namely Simpson Grade IV, with wide variations in the volume and location of residual tumors, making it complicated to evaluate the achievement of surgical goals and predict the prognosis of these tumors. Authors of the present study investigated the factors related to necessity of retreatment and tried to identify any surgical nuances achievable with the aid of modern neurosurgical techniques for meningiomas treated using Simpson Grade IV resection.METHODS: This retrospective analysis included patients with WHO Grade I meningiomas treated using Simpson Grade IV resection as the initial therapy at the University of Tokyo Hospital between January 1995 and April 2010.