The purpose of the present study was to examine the prevalence and predictors of complementary and alternative medicine (CAM) use as well as parental perceptions of CAM efficacy in a large, geographically diverse sample of children with Autism Spectrum Disorders (ASD). Data were obtained from a web-based survey administered to parents of children with ASD at four sites participating in the Mental Health Research Network (MHRN). The web survey obtained information about services and treatments received by children with ASD as well as the caregivers’ experiences with having a child with ASD. Approximately 88% of the sample had either used CAM in the past or had recently used some type of CAM. The following characteristics were associated with CAM use: greater parental education, younger child age, a mix of regular and special classroom settings and prescription drug use in the past three months. The use of CAM was very prevalent in this large, geographically diverse sample of children with ASD. It is critical that providers be prepared to discuss the advantages and potential side effects with families to help them make well-informed health care decisions and prevent possible CAM-drug interactions.