Objective: To determine the extent to which scores on the Conners' Continuous Performance Task, 2nd edition (CCPT-II) predict classification of ADHD in a mixed clinical sample. Method: The sample contained 77 participants: 20 were diagnosed ADHD and 57 with either Major Depressive Disorder or an Anxiety Disorder (mean age = 9.67; 61.8% male; 88.8% right handed; average education = 3.95 years). Anxiety and depression were chosen because these disorders are known to affect attention and concentration. Clinical diagnoses were based on Diagnostic and Statistical Manual for Mental Disorders-IV-TR (DSM-IV-TR) criteria. T-scores obtained from the CCPT-II were used as measures of sustained attention, impulsivity, and response variability. Results: Analyses were conducted at the 0.01 significance level. A discriminant function analysis was conducted for ADHD group membership using CCPT-II T-scores as predictors. Univariate ANOVAs revealed that groups differed significantly on the Omissions, Hit Rate, Hit Rate Standard Error, and Variability scores. A single discriminant function was calculated and the value was significantly different for the groups (chi-square = 24.589, df = 4, p = .017). Correlations between predictor values and the discriminant function suggest that high Variability scores are the best predictor for ADHD group membership. Overall the discriminant function successfully predicted group membership for 80.5% of cases. Conclusion(s): Children with ADHD differed significantly from children with anxiety and depression on the CCPT-II. As these disorders often manifest with impairment in concentration, assessment of abilities is necessary. Although only 80.5% of cases were successfully predicted, scores from this test can provide objective evidence for an accurate diagnosis.