  We describe a measurement of the W boson mass mW using 200/pb of root-s =
1.96 TeV p-pbar collision data taken with the CDF II detector. With a sample of
63,964 W -> e nu candidates and 51,128 W -> mu nu candidates, we measure mW =
[80.413 +- 0.034 (stat) +- 0.034 (sys) = 80.413 +- 0.048] GeV/c^2. This is the
single most precise mW measurement to date. When combined with other measured
electroweak parameters, this result further constrains the properties of
unobserved particles coupling to W and Z bosons.
