Abstract A random population sample of 1479 Chinese boys from Hong Kong was screened and diagnosed in a two-stage epidemiological study. Four groups, age 7–8, were distinguished: (1) a pure hyperactive group (HA), (2) a mixed hyperactive/conduct–disordered group (HA + CD), (3) a pure conduct-disordered group (CD), and (4) a normal control group (N). On a visual search task, only the HA children showed a specific processing deficit in performance. This confirms the diagnostic value of such a deficit for hyperactivity, differentiating it from conduct disorder. The failure to find a similar deficit in the HA + CD group raises questions concerning the clinical identity of these children. Each group showed a performance decrement over time in the visual search task but the decrement did not differ between the four groups. This observation is not congruent with the reports of a short attention span in hyperactive children; explanations of this apparent contradiction are considered.