We present a measurement of the fraction f+ of right-handed W bosons produced in top quark decays, based on a candidate sample of t[overline t] events in the lepton+jets decay mode. These data correspond to an integrated luminosity of 230 pb-1, collected by the D0 detector at the Fermilab Tevatron p[overline p] Collider at sqrt(s)=1.96 TeV. We use a constrained fit to reconstruct the kinematics of the t[overline t] and decay products, which allows for the measurement of the leptonic decay angle theta* for each event. By comparing the costheta* distribution from the data with those for the expected background and signal for various values of f+, we find f+=0.00±0.13(stat)±0.07(syst). This measurement is consistent with the standard model prediction of f+=3.6×10-4.