  We discuss theoretical uncertainties of the measurement of the $W$ boson mass
at LEP2 energies, reconstructed with the help of the tandem of the Monte Carlo
event generators KoralW and YFSWW3. Exploiting numerical results obtained with
these programs, and the existing knowledge in the literature, we estimate that
the theoretical uncertainty of the $W$ mass due to electroweak corrections, as
reconstructed at LEP2 with the help of these programs, is $\sim 5 $MeV. Since
we use certain idealized event selections and a simple $M_W$-fitting procedure,
our numerical exercises can be (should be) repeated for the actual ``$M_W$
extraction methods'' of the LEP2 measurements, using KoralW and YFSWW3 or other
Monte Carlo programs.
