The accuracy of GW in the determination of defect energy levels is assessed through calculations on a set of well-characterized point defects in semiconductors: the As antisite in GaAs, the substitutional Mg in GaN, the interstitial C in Si, the Si dangling bond, and the Si split-vacancy complex in diamond. We show that the GW scheme achieves a reliable description of charge transition levels, but the overall accuracy is comparable to that of hybrid-functional calculations.