The aim of the study was to test whether sustained attention and vigilance, often considered as the same phenomenon, dissociate on the Conners' Continuous Performance Test, and whether subjects with ADHD-Inattentive type and ADHD-Combined type differ with regard to these measures. Sixty-five healthy controls and 67 subjects with ADHD between 9 and 16 years of age participated in the study. The ADHD-I group performed below control children on Hit Reaction Time Block Change, considered to measure sustained attention and the ADHD-C group scored below controls on Hit Reaction Time Inter-Stimulus-Interval, considered to measure vigilance. Comparing the two clinical groups showed a test by group interaction, with ADHD-I subjects performing below ADHD-C subjects with regard to sustained attention and above ADHD-C subjects with regard to vigilance. Sustained attention on the CCPT correlated specifically with parent and teacher ratings of inattention, but not with ratings of hyperactivity-impulsivity, while vigilance correlated with all symptom ratings.