Children aged 5-13 years with DSM-III diagnoses of Attention Deficit (ADDH), Anxiety, (ANX), or Conduct plus ADDH (HC) Disorder and matched normal controls were compared on a set of laboratory measures of impulsivity, arousal, motor performance, activity level, and cognition, and on behavior ratings during testing. While ANX patients did not differ from their controls, ADDH and HC patients did on Verbal IQ, most of the behavior ratings, and on about one-third of the test variables. ANX patients were about 1 year older, and more likely to be female, than ADDH and HC patients. When age, sex, and verbal IQ effects were partialed out, very few differences among the three diagnostic groups remained. The importance of precise control of such variables is emphasized and the impact of the failure to do so in past studies is discussed. The question is raised whether the deficit in verbal IQ is not so much a defect of matching as the essential feature of ADDH from which most of the other commonly reported cognitive symptoms stem.