The first measurement of the production cross section for a single top quark in association with a W boson with the CMS experiment at the Large Hadron Collider is presented. The analysed data is from proton-proton collisions at a centre-of-mass energy of 7 TeV, corresponding to an integrated luminosity of 4.9 fb-1. The analysis focuses on the leptonic final states of the process and implements a cut-based method. The final state consists of two leptons, two neutrinos and a jet from the fragmentation of a b-quark. The primary source of background events arises from tt(overbar) production, followed by Z/gamma* processes. The observed signal has a significance of 3:5sigma and corresponds to a cross section measurement of 14:9+5:0 -5:1 (stat + syst) pb, in agreement with the Standard Model expectations.