The aim of the present study was to investigate the clinical usefulness of a semi-structured diagnostic parent interview, i.e., the Kiddie-Disruptive Behavior Disorder Schedule (K-DBDS), in preschool children. For Oppositional Defiant Disorder (ODD), to define symptoms two coding methods were compared, i.e., one based on the threshold "often" and the other based on the frequency of behaviors in combination with the presence of clinical concern. For Attention-Deficit/Hyperactivity Disorder (ADHD), to define symptoms, two coding methods were compared, i.e., one with and one without consideration of pervasiveness across contexts. Participants were referred preschool children with externalizing behavioral problems (N = 193; 83% male) and typically developing (TD) children (N = 58; 71% male). The referred children were given a diagnosis of either ODD/CD (N = 39), or ADHD (N = 58) or comorbid ODD/CD+ADHD (N = 57) or no diagnosis (N = 39) based on best-estimate diagnosis. Receiver Operating Characteristic curve analyses showed that a cutoff score of four ODD symptoms using "often" as the threshold for frequency of behaviors led to a sensitivity of 87% and a specificity of 93%; the coding method which included the frequency of behaviors yielded a sensitivity of 56% and a specificity of 100%. For ADHD, a clinical cutoff score of five symptoms without the pervasiveness criterion yielded a sensitivity of 83% and a specificity of 98%; when the pervasiveness criterion was included sensitivity was 77% and specificity 98%. In the clinical assessment of ODD and ADHD in preschool children, the K-DBDS may be used with ODD symptom definition based on the threshold "often" and ADHD pervasiveness across contexts not included.