Inter-rater agreement about children's symptoms of attention-deficit/hyperactivity disorder (ADHD) is crucial for accurately identifying and treating children with this condition. Decades of empirical research demonstrate that parents and teachers rarely agree about children's ADHD symptoms, yet few studies have tested the factors that relate to parent-teacher agreement. This exploratory study examined potential associations between parent-teacher agreement about child ADHD symptoms, and child factors (academic functioning, demographic characteristics, externalizing psychopathology, and social functioning), in a community (n= 752; mean age = 7.28; 51.3% male; 46.5% White) and in a clinical (n= 213; mean age = 8.58; 69.0% male; 70.4% White) sample. Agreement was examined using polynomial regression analyses, which overcome mathematical limitations and constraints imposed by using difference scores. Parent-teacher agreement about ADHD symptoms related to several academic and social functioning variables in the community sample. Most relationships were non-linear (e.g., quadratic, cubic). The 3-dimensional distribution of the results revealed that parent-teacher agreement was strongest for children functioning approximately 1 standard deviation below the mean (but not lower) on grades, academic enablers, academic performance, and social functioning. In the clinical sample, only teacher-rated social functioning related to parent-teacher agreement about ADHD symptoms. These findings provide a more nuanced understanding of parent-teacher agreement, thereby advancing theoretical knowledge. An implication for assessment and treatment is that children with very poor, or conversely, very good, social and academic functioning are at risk for parent-teacher disagreement on ADHD symptom ratings. [This is the online version of an article published in "Journal of Psychopathology and Behavioral Assessment."]