Predictability problem for two-dimensional decaying turbulence is addressed by means of numerical simulations. Qualitative and quantitative comparisons with previous results obtained by closure approximations are critically examined. It is found that, as for other features of two-dimensional turbulence, the role of coherent vortices is essential for a correct interpretation of the results. A Lagrangian, vortex-based, definition for the growth of incertitudes leads in general to an enhancement of the predictability time. ©1997 American Institute of Physics.