The WHO 2007 classification of tumors of the CNS distinguishes between diffuse astrocytoma WHO grade II (A IIWHO2007) and anaplastic astrocytoma WHO grade III (AA IIIWHO2007). Patients with A IIWHO2007 are significantly younger and survive significantly longer than those with AA IIIWHO2007. So far, classification and grading relies on morphological grounds only and does not yet take into account IDH status, a molecular marker of prognostic relevance. We here demonstrate that WHO 2007 grading performs poorly in predicting prognosis when applied to astrocytoma carrying IDH mutations. Three independent series including a total of 1360 adult diffuse astrocytic gliomas with IDH mutation containing 683 A IIIDHmut, 562 AA IIIIDHmut and 115 GBMIDHmut have been examined for age distribution and survival. In all three series patients with A IIIDHmut and AA IIIIDHmut were of identical age at presentation of disease (36–37 years) and the difference in survival between grades was much less (10.9 years for A IIIDHmut, 9.3 years for AA IIIIDHmut) than that reported for A IIWHO2007versus AA IIIWHO2007. Our analyses imply that the differences in age and survival between A IIWHO2007 and AA IIIWHO2007 predominantly depend on the fraction of IDH-non-mutant astrocytomas in the cohort. This data poses a substantial challenge for the current practice of astrocytoma grading and risk stratification and is likely to have far-reaching consequences on the management of patients with IDH-mutant astrocytoma.