Intrinsic and extrinsic stimuli result in profound pituitary growth changes ranging from hypoplasia to hyperplasia. Pituitary tumor transforming gene (PTTG) abundance correlates with pituitary trophic status. Mice with Pttg inactivation exhibit pituitary hypoplasia, whereas targeted pituitary PTTG overexpression driven by α-subunit glycoprotein (αGSU) promoter results in focal pituitary hyperplasia. To test the impact of pituitary hyperplasia on tumor development, we crossbred αGSU.PTTG with Rb+/− mice, which develop pituitary tumors with high penetrance. Pituitary glands of resulting bitransgenic αGSU.PTTGxRb+/− mice were compared with monotransgenic αGSU.PTTG, Rb+/−, and wild-type mice. Confocal microscopy showed that PTTG-overexpressing cells have enlarged nuclei and marked redistribution of chromatin, and electron microscopy of αGSU.PTTG pituitaries showed enlarged gonadotrophs with prominent Golgi complexes and numerous secretory granules. These morphological findings were even more remarkable in αGSU.PTTGxRb+/− pituitaries. Mice from all four genotypes were sequentially imaged by magnetic resonance imaging to evaluate pituitary volume, and glands from αGSU.PTTGxRb+/− mice were the largest as early as 2 months of age (P = 0.0003). Cumulative incidence of pituitary tumors visualized by magnetic resonance imaging did not differ between Rb+/− and αGSU.PTTGxRb+/− mice. However, anterior lobe tumors determined after necropsy were 3.5 times more frequent in αGSU.PTTGxRb+/− than in Rb+/− mice (P = 0.0036), whereas the frequency of intermediate lobe tumors was similar. In summary, αGSU.PTTGxRb+/− pituitary glands exhibit enhanced cellular activity, increased volume, and higher prevalence of anterior pituitary tumors, indicating that changes in pituitary PTTG content directly relate to both pituitary trophic status and tumorigenic potential.