We describe a measurement of the charge asymmetry of leptons from W
boson decays in the rapidity range 0 < |y_l| < 2.5 using W -> e nu, mu nu
events from 110 +/- 7 pb^-1 of data collected by the CDF detector during
1992-95.  The asymmetry data constrain the ratio of d and u quark momentum
distributions in the proton over the x range of 0.006 to 0.34 at Q^2 \approx
M^2_W.  The asymmetry predictions that use parton distribution functions
obtained from previously published CDF data in the central rapidity region (0
< |y_l| < 1.1) do not agree with the new data in the large rapidity region
(|y_l > 1.1).