Prior nonexperimental studies have been used to conclude that children's reading and mathematics achievement bidirectionally influence each other over time, with strong paths from (a) early reading to later mathematics and (b) early mathematics to later reading. In the most influential study on the topic, the early math-to-later-reading path was reported to be stronger than the early reading-to-later-math path (Duncan et al., 2007). Yet prior estimates may be confounded by stable environmental and personal factors influencing both reading and mathematics achievement. We reexamined the bidirectional relations between reading and mathematics achievement using both traditional models and extensions intended to account for unmeasured confounding. Results based on a large nationally representative sample of children from kindergarten to 3rd grade (N = 9,612) indicated that the estimated effects between reading and mathematics achievement differ substantially after accounting for the confounding effects of stable unmeasured factors. In these models, autoregressive and cross-lagged paths were substantially reduced. The finding that early mathematics predicts later reading more strongly than early reading predicts later math disappears and sometimes reverses, suggesting that larger paths from math to reading than from reading to math in previous related analyses are not causally informative. Stability in early mathematics and reading achievement resulted from substantially overlapping time invariant factors that correlate above .90. (PsycInfo Database Record (c) 2020 APA, all rights reserved).