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Object Recently the relevance of Simpson resection grade as a prognostic factor for recurrence of WHO Grade I meningiomas was challenged, contradicting many previous scientific reports and traditional neurosurgical teaching. The objective of this study was to determine whether the predictive value of Simpson resection grade is outdated or remains valid with respect to meningioma recurrence and overall survival. Methods All patients at least 16 years old who underwent primary craniotomies for convexity meningiomas at Oslo University–affiliated hospitals (Rikshospitalet and Ullevål University Hospitals) in the period between January 1, 1990, and January 27, 2011, were included. Overall survival and retreatment-free survival rates were correlated with patient- and surgery-specific factors. Results Three hundred ninety-one consecutive patients were included in the study. The median patient age was 60.1 years (range 19–92 years). The female-to-male ratio was 2.1:1. The WHO grades were Grade I in 353 (90.3%), Grade II in 22 (5.6%), and Grade III in 16 (4.1%). The follow-up rate was 100%. Median follow-up time was 7.1 years (range 0.0–20.9 years) and total observation time was 3147 patient-years. The 1-, 5-, and 10-year overall survival rates were 96%, 89%, and 78%, respectively. Age, sex, WHO grade, and Simpson grade were significantly associated with overall survival. The 1-, 5-, and 10-year retreatment-free survival rates were 99%, 94%, and 90%, respectively. Simpson resection grade and WHO grade were significantly associated with retreatment-free survival. The hazard ratios for retreatment after combined Simpson resection Grades II+III and IV+V were 4.9- and 13.2-times higher than after Simpson Grade I resection, respectively. Conclusions Simpson Grade I resection should continue to be the goal for convexity meningiomas.