Adolescents' (n = 342, 169 boys) general algebra and algebra word problems performance were assessed in 9th grade as were intelligence, academic achievement, working memory, and spatial abilities in prior grades. The adolescents reported on their academic attitudes and anxiety and their teachers reported on their in-class attentive behavior in 7th to 9th grade. There were no sex differences on the general algebra measure or for mathematics achievement, but boys had an advantage on the algebra word problems measure (d = .51) and for spatial abilities (ds = .29 to .58). Boys had higher mathematics self-efficacy (d = .24 to .33), lower mathematics anxiety (ds = −.31 to −.53) and were less attentive in classrooms (ds = −.28 to −.37). A series of structural equation models revealed the sex difference for algebra word problems was mediated by spatial abilities and mathematics anxiety, controlling myriad confounds. Public Significance StatementSex differences in mathematics are typically small but larger for word problems. The latter assess the ability to use mathematics in problem-solving situations and are often included on high-stakes tests. Boys' higher spatial abilities appeared to provide them with an advantage and girls' mathematics anxiety a disadvantage in solving algebra word problems.