Data validating the Autism Parenting Stress Index (APSI) is presented for 274 children under age six. Cronbach’s alpha was .827. As a measure of parenting stress specific to core and co-morbid symptoms of autism, the APSI is unique. It is intended for use by clinicians to identify areas where parents need support with parenting skills, and to assess the effect of intervention on parenting stress. Mean parenting stress in the autism group was four times that of the typical group and double that of the other developmental delay group [F(2,272) = 153; p < 001]. An exploratory factor analysis suggested three factors impacting parenting stress: one relating to core deficits, one to co-morbid behavioral symptoms, and one to co-morbid physical symptoms.