This paper reports on the effects the use of a pocket calculator-based computer algebra system (CAS) has on the performance in mathematics of grade 11 students in Germany. A project started at 8 of about one hundred upper secondary schools in the federal state of Thuringia in 1999; 3 years later the former restrictions on the use of technology in math education were lifted. In 2004, more than a quarter of all Thuringian upper secondary schools used CAS in math classes. Beginning in 2000, a test was carried out each year to compare the performance of CAS and non-CAS students (from different control schools). In 70% of the cases CAS students performed better than, and in the remaining 30% they performed as well as, non-CAS students. There is evidence that students in advanced courses benefit more from using CAS than students in basic courses. (Contains 7 tables and 7 figures.)