Robust findings demonstrate that parents of children with autism spectrum disorder (ASD) experience high levels of parenting stress that are associated with negative outcomes for families. Although the majority of research on parenting stress has focused on parents of children with an existing diagnosis, parents of young children with concerns about ASD-related behaviors also face many unique challenges before the time of diagnosis. However, no study to date has examined patterns of parenting stress among parents of children with ASD concerns prior to a formal ASD diagnosis. Therefore, the current study investigated longitudinal trajectories of parenting stress among parents of young children with ASD concerns compared to parents of children with non-ASD developmental concerns (e.g., language delay), and parents of children with no developmental concerns. Known predictors of parenting stress were also examined. Results from multilevel model analyses revealed that parents of children with ASD concerns experienced consistently higher levels of parenting stress across early child development compared to parents of children with non-ASD developmental concerns and those with no concerns. Additionally, parenting efficacy, psychological functioning, social satisfaction, and child social communication behaviors predicted levels of parenting stress for all parents. Autism Res 2020, 13: 82-92. © 2019 International Society for Autism Research, Wiley Periodicals, Inc. LAY SUMMARY: This study examined parenting stress across time among parents of young children with ASD concerns before receiving a diagnosis. Parents of children with ASD concerns reported consistently higher levels of parenting stress compared to parents of children with other developmental concerns and parents of children with no concerns. Also, ASD concerns predicted parenting stress in addition to other parent and child predictors of parenting stress. These findings highlight the need to better support families before an ASD diagnosis.