The 238U(n,f) cross section has been measured from 3 eV to ≃ 100 keV with the Rensselaer Intense Neutron Spectrometer, a 75-ton lead slowing down spectrometer at the Gaerttner Laboratory at Rensselaer Polytechnic Institute. Four fission ionization chambers containing a total of ≃ 0.8 g of 238U (4.1 ppm 235U) were used for the measurements. The fission widths of the 6.67-, 20.9-, and 36.8-eV resonances were measured as (10 ± 1), (58 ± 9), and (12 ±2) neV, respectively. By combining these fission results and published resonance parameters, the 238U thermal fission cross-section contribution from positive energy resonances was determined to be (2.7 ± 0.3) µb. The resonance fission integral from 0.4 eV to 100 keV was determined to be (1.30 ±0.15) mb.