  This letter reports a measurement of the muon charge asymmetry from $W$
bosons produced in proton-proton collisions at a centre-of-mass energy of 7 TeV
with the ATLAS experiment at the LHC. The asymmetry is measured in the $W
\rightarrow \mu\nu$ decay mode as a function of the muon pseudorapidity using a
data sample corresponding to a total integrated luminosity of 31 pb$^{-1}$. The
results are compared to predictions based on next-to-leading order calculations
with various parton distribution functions. This measurement provides
information on the $u$ and $d$ quark momentum fractions in the proton.
