Research has identified a gender gap in the mathematical attainment of post-primary students around the world, favouring male students. In Ireland, following a review of the outcomes of a high-stakes examination taken by students at the end of post-primary schooling over an 18- year period, a similar such gap has been identified here and is widening. Data are presented to show that this gender gap widened with the introduction of a revised post-primary mathematics curriculum, colloquially known as Project Maths. This paper explores potential reasons behind the widening gap. Problem solving appears to be the pivotal issue and spatial ability may be a contributory factor. Addressing students’ spatial ability is explored as way to address the gender gap and enable students to reach their full mathematical potential.