Evidence is presented for the associated production of a single top quark and W boson in pp collisions at √s=7  TeV with the CMS experiment at the LHC. The analyzed data correspond to an integrated luminosity of 4.9  fb−1. The measurement is performed using events with two leptons and a jet originated from a b quark. A multivariate analysis based on kinematic properties is utilized to separate the tt¯ background from the signal. The observed signal has a significance of 4.0σ and corresponds to a cross section of 16+5−4  pb, in agreement with the standard model expectation of 15.6±0.4+1.0−1.2  pb.