The mammalian target of rapamycin (mTOR) signaling pathway is a key developmental pathway involved in mechanisms underlying cellular aging and neurodegeneration. We hypothesized that its deregulation may occur during early brain development in patients with Down syndrome (DS). The expression patterns and cellular distribution of components of mTOR signaling (phosphorylated S6, phosphorylated S6 kinase, phosphorylated eukaryotic initiation factor 4E binding protein 1, and phosphorylated mTOR) were investigated in developing hippocampi from controls and patients with DS and from adults with DS and Alzheimer disease–associated pathology using immunocytochemistry. In control hippocampi, only phosphorylated S6 was detected prenatally (19–41 gestational weeks); it became undetectable 2 months postnatally. Increased expression of phosphorylated S6, phosphorylated S6 kinase, phosphorylated eukaryotic initiation factor 4E binding protein 1, and phosphorylated mTOR was observed in DS hippocampus compared with controls. Phosphorylated S6 and phosphorylated S6 kinase were detected prenatally and persisted throughout postnatal development. Prominent expression of mTOR components was observed in pyramidal neurons with granulovacuolar degeneration and in neurons containing neurofibrillary tangles in the hippocampi of DS subjects with Alzheimer disease pathology. These findings suggest that a dysregulated mTOR pathway may contribute to both early hippocampal developmental abnormalities and hippocampal functional impairment developing before neurodegeneration. Moreover, the expression patterns of mTOR components in adult DS hippocampus support its association with Alzheimer disease–related histopathologic changes.