Two independent cohorts (N = 155, N = 126) of infants at high and low risk for autism spectrum disorder (ASD) were followed prospectively between 6 and 36 months of age, when n = 46 were diagnosed with ASD. Gaze to adult faces was coded--during a developmental assessment (Cohort 1) or a play interaction (Cohort 2). Across both cohorts, most children developing ASD showed sharp declines in gaze to faces over time, relative to children without ASD. These findings suggest that declining developmental trajectories may be more common than previously recognized by retrospective methods. Trajectory-based screening methods could potentially identify children in the early stages of symptom onset and allow for early intervention before the full disorder has developed.