The Processing Speed Index (PSI) was first introduced on the Wechsler Intelligence Scale, Third Edition (WISC-III; D. Wechsler, 1991), and little is known about its clinical significance. In a referred sample (N = 980), children with neurological disorders (ADHD, autism, bipolar disorder, and LD) had mean PSI and Freedom from Distractibility Index (FDI) scores that were below the group mean IQ and lower than Verbal Comprehension (VCI) and Perceptual Organization (POI). For these groups, Coding was lower than Symbol Search. The majority of these children had learning, attention, writing, and processing speed weaknesses. This pattern was not found in the other clinical groups. For children with depression, only PSI was low. Children with anxiety disorders, oppositional-defiant disorder, and mental retardation had no PSI weakness. PSI and POI were both low in children with traumatic brain injury and spina bifida. Implications for a revision of the WISC-III (WISC-IV; D. Wechsler, 2003) are discussed. © 2005 Wiley Periodicals, Inc. Psychol Schs 42: 333–343, 2005.