The unequal prevalence of hyperactivity in Britain and the U.S. was investigated with a cross-national diagnostic study. Case histories of 6-11-yr-old boys were evaluated by British and American research teams as well as British and American clinician panels using both ICD-9 and DSM-III. Interrater agreement was acceptably high only for the specially trained research teams. ICD-9 generated fewer diagnoses of Hyperkinetic Syndrome than did DSM-III of Attention Deficit Disorder with Hyperactivity. The difference was greatest for U.K. clinicians. Diagnostic scheme and clinician training both contribute to the difference in reported rates.