Fragile X syndrome (FXS) is the most common single gene (FMR1) disorder affecting cognitive and behavioral function in humans. This syndrome is characterized by a cluster of abnormalities including lower IQ, attention deficits, impairments in adaptive behavior and increased incidence of autism. Here, we show that young males with FXS have profound deficits in prepulse inhibition (PPI), a basic marker of sensorimotor gating that has been extensively studied in rodents. Importantly, the magnitude of the PPI impairments in the fragile X children predicted the severity of their IQ, attention, adaptive behavior and autistic phenotypes. Additionally, these measures were highly correlated with each other, suggesting that a shared mechanism underlies this complex phenotypic cluster. Studies in Fmr1-knockout mice also revealed sensorimotor gating and learning abnormalities. However, PPI and learning were enhanced rather than reduced in the mutants. Therefore, these data show that mutations of the FMR1 gene impact equivalent processes in both humans and mice. However, since these phenotypic changes are opposite in direction, they also suggest that murine compensatory mechanisms following loss of FMR1 function differ from those in humans.