This article reports the results of the first systematic, though only exploratory, study that assesses the effectiveness of the Philosophy for Children (commonly known as P4C) programme in promoting children's critical thinking in Hong Kong. Forty-two Secondary 1 students volunteered for this study, from whom 28 students were randomly selected and randomly assigned to 2 groups of 14 each: one receiving P4C lessons and the other receiving English lessons. The students who were taught P4C were found to show a greater improvement in the reasoning test performance than those who were not, to be capable of doing philosophy, and to have a positive attitude towards doing philosophy in the classroom. It was also found that P4C played a major role in developing the students' critical thinking, and that 10 crucial factors contributed to the success of the P4C programme. (Contains 3 tables.)