Background: The symptoms of attention-deficit/hyperactivity disorder (ADHD) are non-specific, and a range of possible causes and comorbidities need to be considered in children referred for assessment. Objective: To examine the factors associated with ADHD diagnosis following multidisciplinary assessment. Methods: Children underwent multidisciplinary evaluation including parent and teacher questionnaires; semi-structured interview to screen for internalizing and externalizing diagnoses; paediatric, psychology and special education assessments; and case conference. Predictors of ADHD diagnosis were examined in univariable and multivariable logistic regression models. Results: Data from 190 assessments (82% male, mean age 6.8 years) were included. ADHD was diagnosed in 132 (70%) cases, of which 77% had one or more comorbidities. In children not diagnosed with ADHD, 60% had one or more alternate diagnosis made. Teacher-reported ADHD symptom severity and learning difficulties were the strongest predictors of ADHD diagnosis. The pattern of comorbid/alternative diagnoses was similar between those diagnosed with ADHD and those not diagnosed with ADHD. Conclusions: Direct report from teachers is the most critical element of the clinical dataset for the evaluation for ADHD. These findings emphasize the importance of cross-situational impairment to ADHD diagnosis. The frequency and similarity of diagnoses in both groups highlight the overlapping nature of childhood developmental disorders, and the importance of evaluating for comorbid disorders regardless of the primary diagnosis.