Abstract: We present a measurement of the W boson helicity fractions in $t\bar{t}$ events with the full dataset collected by the CMS detector in 2011 at a center-of-mass energy of $\sqrt{s}=7$~TeV. This corresponds to an integrated luminosity of $4.5\;\mathrm{fb}^{-1}$. The analysis is based on the dilepton decay channel of $t\bar{t}$ quark pairs, in which both W bosons decay leptonically. Only events containing electrons and/or muons in the final state are considered.