Parents’ concerns about their children’s development were examined prospectively at 14, 24, and 36 months for 89 younger siblings of a child with autism. Parent reported concern was high at all ages (40–75%) and was higher at 24 and 36 months in children with ASD than non-ASD outcomes (p < .05). Communication concerns were reported most frequently. Parent concern compared to impairment classification based on concurrent standardized tests provided better specificity than sensitivity, and was better for communication than social functioning. Parent communication concern (but not social concern) at 24 months and 36 months predicted ASD versus non-ASD outcome; however, children’s impairment on standardized tests yielded greater predictive value at all ages (p < .001). Close monitoring of this at risk group is warranted.