Mutations at the codon 132 in the isocitrate dehydrogenase 1 (IDH1) gene occur early, with a high frequency, in World Health Organization (WHO) grade II gliomas. We investigated the impact of IDH1 mutations on spontaneous glioma growth rate, known to be an early prognostic factor.The mean tumor diameter was assessed on the first MRI performed at diagnosis and on a second MRI, performed immediately before surgery, in a series of 64 WHO grade II gliomas. The patients did not undergo treatment before surgery. Because of a frequent association, we jointly analyzed the 1p19q co-deletion and IDH1 mutations effects on tumor velocity of diameter expansion (mm/year) during preoperative spontaneous growth period. 1p19q co-deletion had a significant slowing effect (p = 0.0133) on tumor growth estimated at -1.7760 ± 0.711 mm/year (95% CI -3.154, -0.366), whereas IDH1 mutations estimated effect of ?0.036 ± 0.833 mm/year (95% CI -1.668; ?1.596) was not significant (p = 0.9654). Our results provide first evidence that IDH1 mutations are not significantly involved in tumor growth rate. By contrast, we confirm that 1p19q co-deletion decreases growth velocity.