The current study examined the incremental validity of the clinical clusters from the Woodcock-Johnson III Tests of Cognitive Abilities (WJ-III COG) for predicting scores on the Woodcock-Johnson III Tests of Achievement (WJ-III ACH). All participants were children and adolescents (N = 4,722) drawn from the nationally representative WJ-III standardization sample. Hierarchical multiple regression analyses were used to assess for cluster-level effects after controlling for the variance accounted for by the General Intellectual Ability (GIA) composite score. Consistent with previous studies, the GIA accounted for clinically significant portions of WJ-III ACH score variance in all of the regression models with R[superscript 2] values ranging from 0.33 to 0.63. The clinical cluster scores collectively accounted for small to moderate incremental effects with no meaningful effects observed for individual indicators. Potential implications of these results for empirically supported interpretation of the WJ-III COG are discussed.