- Daydreaming, a common mental activity, can be excessive and accompanied by distress and impaired functioning in daily life. Although currently not formally identified by diagnostic manuals, daydreaming disorder (maladaptive daydreaming-MD) is a clinically well-defined phenomenon. However, research is lacking regarding the diagnostic reliability of MD. Our aims were: (1) to develop diagnostic criteria and a structured interview for MD, (2) to examine the reliability of this measure for distinguishing individuals with and without MD, and (3) to establish an optimal cutoff score for identifying clinical-level MD using an existing self-report measure. Thirty-one individuals who met screening criteria for MD and 31 matched controls completed the self-report measure and participated in two structured clinical interviews. Each participant was interviewed independently by two clinicians blind to the participant’s group membership. Cohen’s kappa values for the agreement rate between each interviewer and the screening criterion, and between the two interviewers, ranged from good to excellent (k=.63-.84). A cutoff score of 50 on the self-report measure yielded nearly perfect sensitivity and specificity and good-to- excellent agreement between the self-report measure and the interview (k=.68-.81). Our interviews were conducted over the Internet, rather than in person; results might have been influenced by self-selection; and interviewing wider samples is warranted. We found that MD can be diagnosed reliably using a structured interview developed for that purpose. The new diagnostic interview showed excellent agreement with a self- report measure for the disorder. Additionally, we identified a useful cutoff score identified for future self-report research.