The 238U/235U isotope ratio has long been considered invariant in meteoritic materials (i.e., 137.88). This assumption is a cornerstone of the high-precision Pb-Pb dates that define the absolute age of the Solar System. Calcium-aluminum-rich inclusions of the Allende meteorite display variable 238U/235U ratios, ranging between 137.409±0.039 and 137.885±0.009. This range implies substantial uncertainties in the ages previously determined by Pb-Pb dating of CAIs, which may be overestimated by several million years. The correlation of U isotope ratios with proxies for Cm/U (i.e., Th/U and Nd/U) provides strong evidence that the observed variations of 238U/235U in CAIs were produced by the decay of extant 247Cm to 235U in the early Solar System, with an initial 247Cm/235U of ~ 1.1 to 2.4 × 10-4.