The relationship between children's cognitive developmental level, as assessed by Piagetian tasks, and their performance on measures of intellectual and neuropsychological functioning was studied. Eight female and 22 male children--ages 8 to 12 years (mean=10.3)--with negative electroencephalographs and neurologic screening and no history of brain impairment participated in testing as part of a comprehensive evaluation following hospitalization at a psychiatric institution. Children were categorized as being in the "pre-operational,""transitional," or "concrete" stage of cognitive development according to performance on the Concept Assessment Kit-Conservation (Goldschmid and Benter, 1968). The first 10 children in each category were selected as subjects, and were tested with the Luria-Nebraska Neuropsychological Battery-Children's Revision (LNNB-CR; Golden, 1980) and the Wechsler Intelligence Scale for Children-Revised (Wechsler, 1974). Results indicate that the IQ of the children did not differ across the three developmental levels. The three groups did differ significantly concerning the number of elevated LNNB-CR subtests (T-scores greater than or equal to 70), with "pre-operational" children having significantly more elevations than "concrete" children. Chi-square for differences in classification on LNNB-CR as brain damaged (greater than or equal to 3 subtests above critical level) revealed significantly more "pre-operational" than "concrete" children classified as brain damaged. Results suggest that a child's cognitive developmental level may have a significant impact on performance on neuropsychological assessment instruments. Cognitive development should be considered when interpreting results of such instruments. Five tables and one graph present study data. (Author/SLD)