Research on sex differences in autism spectrum disorder (ASD) suggests both higher prevalence and a more easily observable presentation of core ASD symptomology in males, which may lead to sex differences in parental concerns. The current study examined whether sex and diagnosis relate to the timing, number, and types of pre-diagnosis concerns for 669 (N[subscript male] = 468) toddlers who screened at risk for ASD. No sex differences in parents' concerns emerged for toddlers diagnosed with ASD; however, in the overall at-risk sample, parents of boys endorsed ASD symptoms, including restricted and repetitive behaviors, more than parents of girls. Future research should examine why sex differences in pre-diagnosis concerns emerge and how they might impact early diagnosis for at-risk boys versus girls.