Despite the prevalence of studies showing increased stress among mothers of individuals with autism spectrum disorders, few studies have examined general stress among typically developing siblings. This study used an online survey to compare the levels of self-reported stress between adolescent siblings of individuals with autism spectrum disorder and Down syndrome. Sibling of individuals with autism reported significantly more overall stress than did siblings of individuals with Down syndrome, as well as more stress specifically attributed to the brother/sister with autism. The two groups did not differ on perceived social support from family and friends. In linear regression models, the disability group (autism vs Down syndrome) was significantly related to sibling stress above and beyond target child behavior problems, perceived social support, and demographic factors. These results help shed light on the daily experiences of adolescent siblings of individuals with autism and call for more research into potential interventions to address increased stress levels.