In this article we report the findings of a randomised control clinical trial that assessed the impact of a Philosophy for Children program and replicated a previous study conducted in Scotland by Topping and Trickey. A Cognitive Abilities Test was administered as a pretest and a posttest to randomly selected experimental groups and control groups. The students in the experimental group engaged in philosophy lessons in a setting of structured, collaborative inquiry in their language arts classes for one hour per (...) week for a number of weeks. The control group received the standard language arts curriculum in that one hour. The study found that the seventh grade students who had experienced the P4C program showed significant gains relative to those in the seventh grade control group at a high level of statistical significance, but the eighth grade students in the experimental group did not show such gains over the eighth grade control group. It was discovered that the seventh grade teachers started the program early in the school year and continued it for a period of 22 to 26 weeks, while the eighth grade teachers started much later and used the program for only 4 to 10 weeks. Our findings suggest that the P4C program must involve students in activities for a significant period of time before the program shows results, but that a meaningful impact on students’ cognitive abilities can be achieved in about 24 weeks of lessons, less than half the time evidenced by the study by Topping and Trickey. (shrink)