The effect of contextual factors on continuous performance testing was examined by administering a vigilance task to 51 children with ADHD under conditions of examiner presence and absence at different times (i.e., beginning and middle) of an assessment battery. The results showed that the ability to differentiate target stimuli from distractors (d-prime) was related to examiner presence vs. absence, and that response bias (beta), a measure of the subject's carefulness in responding to stimuli, was associated with the time of task administration. The decline in d-prime under conditions of examiner absence was shown to be related more to level of aggression as opposed to level of inattention. Children with ADHD having relatively high levels of aggression demonstrated a significant decline in d-prime scores under conditions of examiner absence, but those with relatively low levels of aggression did not. The results demonstrated the importance of contextual factors on vigilance task performance and suggested a differential impact of contextual variables on children who are inattentive vs. aggressive.