The development of the Asperger Syndrome (and high-functioning autism) Diagnostic Interview (ASDI) is described. Preliminary data from a clinical study suggest that inter-rater reliability and test-retest stability may be excellent, with kappas exceeding 0.90 in both instances. The validity appears to be relatively good. No attempt was made in the present study to validate the instrument as regards the distinction between Asperger syndrome and high-functioning autism.