Attention Deficit Hyperactivity Disorder (ADHD) is one of the most prominent neurobehavioral disorders of childhood that is heavily researched and often revised. Deficits in attention/concentration, impulsivity, inhibition, information processing speed, working memory and executive functioning manifest differently according to subtype diagnosis for both children and adults. As a result, previous attempts to construct a unifying theory of ADHD with neural correlates and quantifiable performance discrepancies have resulted in a proliferation of literature reviews citing both significant and insignificant research findings. The purpose of the current study was to construct a homogenous sample of children diagnosed with ADHD and examine purported subtype differences in working memory and information processing speed using the Wechsler Intelligence Scales for Children - Forth Edition (WISC-IV). Sixty participants were selected from archival data from two clinical sites and separated into three groups according to the current DSM-IV-TR ADHD subtype criteria: ADHD Predominately Inattentive type (ADHD-IN), ADHD Predominately Hyperactive/Impulsive type (ADHD-HY), and ADHD Combined type (ADHD-C). Significant differences within groups were revealed on the Coding subtest and Processing Speed Index (PSI) relative to performance on the Perceptual Reasoning Index (PRI). No significant between groups or interaction effects were revealed. While the goal of the current study was aimed at discovering evidence of greater deficits in processing speed by the ADHD-IN group compared to the other two groups, processing speed was reduced for the entire sample providing more evidence of a possible neurological deficit/basis inherent to ADHD. The results of the current study provided minimal evidence of differences on WISC-IV measures of working memory within the current sample. The current study was successful in correcting previous methodological flaws inherent in the relevant literature by constructing a truly homogenous sample of ADHD and provided strong evidence for the necessity of an accurate diagnosis of ADHD subtypes.