Discriminant analysis of vigilance performance measures differentiated 11 adolescents with attention deficit disorder and hyperactivity from 20 adolescents with conduct disorder, while univariate analyses of variance failed to reveal statistically significant differences between the two groups. While both groups displayed similarities in inhibition deficit, the results indicated that attentional behavior is not identical in these two disorders, especially with respect to variability in performance. This study highlights the importance of multivariate approaches in psychiatric research and suggests that relevant information which is contained in the relationship between variables may escape detection by univariate methods. (Author/CL)