Abstract Preschoolers' cognitive abilities were assessed each year as part of the Head Start Program requirements. The Head Start PK-4 Center evaluated preschoolers' cognition by administering the Learning Accomplishment Profile-Diagnostic (LAP-D), as a pretest and posttest measure. The LAP-D study used archival data collected from the 2009-2010 the pretest and posttest results to conduct a quasi-experimental study focused on a single-group interrupted time-series design. The researcher investigated the differences between the LAP-D Cognitive (matching and counting), gender, age and ethnicity of N= 488 preschoolers. Data analyses were conducted using a multivariate analysis of variance (MANOVA). Findings of the study indicated there was no significant difference from pretest to posttest in cognitive development of preschoolers in the Head Start PK-4 Center. There were differences in cognitive matching and cognitive counting scores by time. Secondly, there were no significant differences, by gender in cognitive development of preschoolers. There were differences in posttest scores by gender and test type (cognitive matching vs. cognitive counting). Thirdly, there were no significant differences, by age, in cognitive development of preschooler's as measured by the LAP-D. The hypothesis was partially rejected. There were differences, by ethnicity, in cognitive development of preschoolers as measured by the LAP-D. There were significant differences in the interaction of ethnicity and posttest scores. Future research recommendations should focus on the Head Start Program developing a universal assessment that measures preschooler's cognitive development. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]