The current study describes the development and psychometric properties of a new measure targeting sensitivity to change of core autism spectrum disorder (ASD) symptoms, the Autism Impact Measure (AIM). The AIM uses a 2-week recall period with items rated on two corresponding 5-point scales (frequency and impact). Psychometric properties were examined using a large sample (n = 440) of children with ASD enrolled in the Autism Treatment Network. The exploratory factor analysis indicated four factors and resulted in a 25-item questionnaire with excellent overall model fit. Test-retest reliability, cross-informant reliability, and convergent validity with other measures of ASD symptoms and overall functioning were strong. The AIM is a reliable and valid measure of frequency and impact of core ASD symptoms.