  This paper describes a new measurement of the flux ratio of positive and
negative muons from cosmic-ray interactions in the atmosphere, using data
collected by the CMS detector at ground level and in the underground
experimental cavern. The excellent performance of the CMS detector allowed
detection of muons in the momentum range from 3 GeV to 1 TeV. For muon momenta
below 100 GeV the flux ratio is measured to be a constant $1.2766 \pm
0.0032(stat) \pm 0.0032(syst)$, the most precise measurement to date. At higher
momenta an increase in the charge asymmetry is observed, in agreement with
models of muon production in cosmic-ray showers and compatible with previous
measurements by deep-underground experiments.
