Based on a sensitivity analysis of decay-heat summation calculations, uncertainties are obtained for the calculated decay heat associated with the thermal fission of 235U and the fast fission of 238U and 239Pu. For typical reactor irradiation times and cooling times >100 s, these uncertainties are <3.5% for 235U, 4% for 238U, and 5% for 239Pu. All calculations are based on ENDF/B-IV data.