Critical Thinking AS level was introduced in schools in 2001. There is much research that shows that the teaching of Critical Thinking (CT) does improve critical thinking skills. However, there is less research which shows whether CT skills can be profitably transferred to other subject domains. The purpose of this research was to investigate whether taking an AS level in CT improves performance in other A levels. Using national examination data from 2005 and 2006 we compared the performance of CT students with those not taking CT. The results of both a basic comparison of mean A level performance and a regression analysis (which accounts for prior attainment) showed a significant and positive effect of taking CT, compared with not doing so. According to the regression results, the difference was equivalent to about one tenth of a grade per A level, although the difference was greater for those achieving a higher CT grade.