Systematic discrepancies between experimental and calculated neutron spectra in different fast-neutron media with high 238U content were observed, allowing the uncertainties in certain 238U neutron cross sections to be considered as the source of such discrepancies. Sensitivity studies, as well as the effect of the former uncertainties on such fast-neutron spectra showed that 238U (n, n′) inelastic scattering is by far the main parameter determining the spectrum over a large energy range. Uranium-238 (n, γ) neutron capture is important only at low energies, below the 238U (n, n′) threshold. The (n,γn′) reaction in 238U using Fricke's data leads to an increase in the discrepancies in neutron spectra though this cross section could be considered negligible according to other arguments. A general least-squares adjustment of the 238U total inelastic cross sections, on the neutron spectrum discrepancies, was carried out between 70 keV and 3.6 MeV, using two different inelastic probability matrices (those of ENDF/B-III and UKNDL-DFN 401), which were chosen among several evaluations of this reaction. The adjusted cross sections obtained in an energy mesh corresponding to lethargy intervals of 0.5 imply a systematic reduction of 20 to 30% relative to the initial values in both files, but they appear to be in good agreement with other recent (coarse-mesh) adjusted data, including the KFK-INR set.