  A search for events with an isolated high-energy lepton and large missing
transverse momentum has been performed with the ZEUS detector at HERA using a
total integrated luminosity of 504 $\mathrm{pb}^{-1}$. The results agree well
with Standard Model predictions. The cross section for production of single $W$
bosons in electron-proton collisions with unpolarised electrons is measured to
be $0.89^{+0.25}_{-0.22}$ (stat.) $\pm{0.10}$ (syst.) pb.
