Background: Deficiency in the adaptive allocation of attention to relevant environmental stimuli is an associated feature of autism spectrum disorder (ASD). Recent evidence suggests that individuals with ASD may be specifically impaired in attentional prioritization of novel onsets. Method: We investigated modulation of attention by novel onset stimuli in 22 children with ASD and 22 age- and IQ-matched typically developing (TD) children using a preview visual search task (Donk & Theeuwes, 2003). In preview search, a subset of search stimuli (old) is presented briefly before the remaining stimuli (new) with the effect that search times for targets appearing among the new elements are typically shorter than for those appearing among the old elements. Results: Whereas the TD group exhibited faster reaction time (RT) to targets occurring as novel search elements, the ASD group performed similarly in target new and old conditions, indicating impaired attentional prioritization of novel onsets. Group differences in eye-movement behavior, including fixation frequency and saccadic error for novel onset stimuli, were consistent with the RT findings. Attentional modulation by novel onsets varied inversely with social-communicative symptom severity in the ASD group. Conclusions: The results provide further evidence of reduced sensitivity to novel onsets in ASD, and suggest that impaired processing of dynamic stimuli, possibly associated with abnormalities in the dorsal visual processing stream, may be implicated in the core symptoms of ASD.