Convergent and discriminant validity of the Mental Processing Scales of the Kaufman Assessment Battery for Children (K-ABC) were examined using 51 first-grade children. Convergent validity was assessed using the Reading Recognition and Comprehension subtests of the Peabody Individual Achievement Test. Discriminant validity was assessed using a measure of anxiety/self-esteem, the Child Anxiety Scale, and a measure of hyperactive behavior, the Hyperactivity scale of the Achenbach Child Behavior Checklist. Results supported the convergent validity of the K-ABC; correlations with reading achievement were fairly large. The discriminant validity received only partial support. The K-ABC did not correlate with the Child Anxiety Scale, but did show rather large correlations with the measure of hyperactive behavior. Implications for understanding what the K-ABC Mental Processing Scales are measuring are discussed.