More than two thirds of the germ cells produced during female embryonic development undergo apoptosis shortly after they are generated. This leaves a reduced number of oocytes within the ovary at birth, and, because new germ cells cannot be made after this point, this limits female fertility and reproductive lifespan. Despite the critical role of apoptosis in controlling the number of available oocytes, the proteins involved have not been fully characterised. Our previous study (Myers et al. Reproduction 2014; 148:211-219) showed that targeted disruption of the Bbc3 gene encoding PUMA (p53 upregulated modulator of apoptosis), a pro-apoptotic protein belonging to the BH3-only subgroup of the BCL-2 family, resulted in an increased number of primordial follicles in the mouse ovary on postnatal day (PN) 10. In this study, we further investigated the role of PUMA in maintaining the primordial follicle pool during puberty and adulthood. Stereological analyses of ovaries showed that Puma-/- mice had significantly more primordial follicles than WT control animals during the transition through puberty (PN20, 30, 40), and females reached sexual maturity with more than twice the number of follicles in their ovarian reserve (PN50:WT 2850±202 vs Puma-/- 6651±307, P<0.0001). Follicle numbers remained elevated during adulthood (PN60, 100 and 200) in Puma-/- mice compared to controls. Furthermore, Puma-/- mice had dramatically increased follicle numbers at PN300, an age when WT mice are nearing the end of their reproductive lifespan because their follicle reserves are significantly depleted (WT 485±198 vs Puma-/- 2257±787, P<0.05). Interestingly, however, preliminary studies suggest that elevated follicle numbers do not confer prolonged fertility in Puma-/- females, possibly due to other reproductive deficits. These findings reveal an important role for PUMA in determining the number of primordial follicles established in the initial ovarian reserve and then subsequently maintained in the ovary throughout reproductive life.