We compare theoretical evolutionary tracks of planetary nebulae nuclei with observational data on over a hundred planetary nebulae in the (log L, log Teff, log texp) space. The simultaneous use of the three coordinates eliminates some interpretation that might be proposed when looking at the (log L, log Teff) plane and at the (Mv, log texp) only. The inconsistencies which we find between theory and observations could be partly removed by adopting a different distance scale.