Comorbid psychopathology and challenging behaviors often occur among persons with developmental disabilities. However, little attention to this issue has been reported in young children. In this study, individuals 17 through 36 months of age with varying diagnoses, such as intellectual disability, developmental delay, and Down syndrome, among others, were evaluated. Greater levels of symptoms of comorbid psychopathology were related to higher rates of challenging behaviors. Additionally, Inattention/Impulsivity, Avoidant Behavior, Anxiety/Repetitive Behaviors, and Tantrum/Conduct Problems were significantly related to high rates of Aggressive and Destructive Behaviors, Stereotypies, and Self-Injurious Behavior. Also, those who displayed greater levels of Eating/Sleep Problems engaged in more Aggressive and Destructive Behaviors and Stereotypies but not Self-Injurious Behavior. Thus, even at young ages, children with atypical development without an autism spectrum disorder diagnosis are at high risk for a variety of problem behaviors and disorders that may be related to one another. (Contains 3 tables.)