At what scale is it possible to observe consistent functional specialization within human prefrontal cortex (PFC), reproducible from one individual to the next? Some studies suggest gross functional divisions between large regions of PFC, but it is not known whether PFC exhibits specialization at the fine-grained scale known to differentiate posterior cortical functions. We used fMRI to confirm a three-way segregation of function between three regions of medial anterior PFC, each centered on coordinates within 15 mm of the other two. Naive participants performed three tasks based on earlier studies, and we investigated activity at regions defined by previous results. In each task, signal was significantly greater at the predicted region than the other two, just millimeters away. These results indicate reproducible functional specialization within PFC, at a much finer scale than previously demonstrated. Furthermore, these findings suggest that divergent results from previous studies may reflect the recruitment of functionally distinct regions and that "reverse inference" should be undertaken with caution.