This one-year longitudinal study investigated the benefits of spatial training among highly gifted science, technology, engineering and mathematics (STEM) undergraduates (28 female, 49 male). Compared to a randomized control condition, 12h of spatial training (1) improved the skills to mentally rotate and visualize cross-sections of 3-D objects shortly after training, (2) narrowed gender differences in spatial skills shortly after training, and (3) improved examination scores in introductory physics (d=.38) but not for other STEM courses. After eight months, however, there were no training differences for spatial skills, STEM course grades, physics self-efficacy, or declared majors. Large gender differences, favoring males, persisted for some spatial skills, physics grades, and physics self-efficacy eight months after training. These results suggest that sustained exposure to spatially enriching activities over several semesters or years may be necessary to address gender gaps in spatial skills among highly gifted STEM undergraduates.