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We present a measurement of the W boson mass using data collected by the DØ. experiment at the Fermilab Tevatron during 1994–1995. We identify W bosons by their decays to en final states where the electron is detected in a forward calorimeter. We extract the W boson mass MW by fitting the transverse mass and transverse electron and neutrino momentum spectra from a sample of 11 089 W→eν decay candidates. We use a sample of 1687 dielectron events, mostly due toZ→ee decays, to constrain our model of the detector response. Using the forward calorimeter data, we measure MW=80.691±0.227 GeV. Combining the forward calorimeter measurements with our previously published central calorimeter results, we obtain MW=80.482 ±0.091 GeV.