Children with Autism Syndrome Disorders (ASDs) exhibit social, communicative, and behavioral deficits. We know that human interaction with dogs, which is thought to serve as a social catalyst, results in a decrease of cortisol levels in healthy adults. Introducing service dogs to children with ASD is an attractive idea that has received growing attention in recent decades. However, no study has measured the physiological impact of service dogs on these children. Therefore, the goal of our study was to assess the effects of service dogs on the basal salivary cortisol secretion of children with ASD. We measured the salivary cortisol levels of 42 children with ASD in three experimental conditions; prior to and during the introduction of a service dog to their family, and after a short period during which the dog was removed from their family. We compared average cortisol levels and Cortisol Awakening Response (CAR) before and during the introduction of the dog to the family and after its withdrawal. We found that the introduction of service dogs translated into a statistically significant diminished CAR. Before the introduction of service dogs, we measured a 58% increase in morning cortisol after awakening, which diminished to 10% when service dogs were present. The increase in morning cortisol jumped back to 48% once the dogs were removed from the families (p<0.05). However, service dogs did not have an effect on the children's average diurnal cortisol levels. These results show that the CAR of children with ASD is sensitive to the presence of service dogs, which lends support to the potential behavioral benefits of service dogs for children with autism.