Basic attentional processes and their impact on developmental trajectories in fragile X syndrome were assessed in a 3-year prospective study. Although fragile X syndrome is a monogenic X-linked disorder, there is striking variability in outcomes even in young boys with the condition. Attention is a key factor constraining interactions with the environment, so it is a perfect candidate to predict trajectories in cognitive and behavioral outcomes. In this study, 48 boys with fragile X syndrome were assessed 3 times over 24 months. Although nonverbal IQ declined, there were significant improvements in nonverbal growth scores and in cognitive attention. In contrast, behavioral difficulties (i.e., autistic symptomatology, hyperactivity–inattention) remained stable over this time frame. Attentional markers in the visual and auditory modalities predicted intellectual abilities and classroom behavior, whereas auditory markers alone predicted autistic symptomatology.