We present the results of a novel early childhood intervention in which disadvantaged 3-4-year-old children were randomized to receive a new preschool and parent education program focused on cognitive and non-cognitive skills (CogX) or to a control group that did not receive preschool education. In addition to a typical academic year (9 month) program, we also evaluated a shortened summer version of the program (2 months) in which children were treated immediately prior to the start of Kindergarten. Both programs, including the shortened version, significantly improved cognitive test scores by about one quarter of a standard deviation relative to the control group at the end of the year. The shortened version of the program was equally as effective as the academic-year program because most of the gains in the academic-year program occurred within the first few months. (author abstract)