Although research has been conducted to support the psychometric properties of rating scales used to assess ADHD in adults, little work has been published examining semi-structured interviews to assess ADHD in adults. The present study examined the test-retest reliability and concurrent validity of the Conners Adult ADHD Diagnostic Interview for Diagnostic and Statistical Manual of Mental Disorders (4th ed.; DSM-IV) in a sample (N = 30) of patients referred to an outpatient clinic. Kappa statistics for individual symptoms of inattention and hyperactivity-impulsivity were in the fair to good range for current report and retrospective childhood report. Kappa values for overall diagnosis, which included all DSM-IV symptoms, were fair for both current (adult) ADHD diagnosis (kappa = .67) and childhood report (kappa = .69). Concurrent validity was demonstrated for adult hyperactive-impulsive symptoms and child inattentive symptoms. The findings are discussed in the context of overall issues pertaining to adult ADHD assessment.