Calculators with computer algebra systems (CAS) are powerful tools when working with equations and algebraic expressions in mathematics. When calculators are allowed to be used during assessments but are not available or provided to every student, they may cause bias. The CAS calculators may also have an impact on the trustworthiness of results. In this study students' use of the CAS calculator in their work with released assessment items from TIMSS Advanced 2008 is studied using two approaches. Eight students familiar with CAS, from two mathematics classes in the 12th form, were video filmed when encouraged to think aloud during their work with the items. In addition, a questionnaire was distributed to all 33 students in the two classes who had been working with a CAS. The main finding is that even if the students are used to working with the CAS calculator, they are not using the calculator to a large extent. The analysis indicates that the difference in performance between the high- and low-achieving students has slightly increased due to the use of the calculator. From a validity perspective one could therefore argue that the CAS calculator is no major threat to the trustworthiness of the assessment. Nevertheless, the result indicates that those students in the study, mainly high achieving, who know how to use the CAS calculator, get an additional advantage. The advantage brings an amount of unfairness into the assessment and could be a threat to the trustworthiness and fairness. (Contains 2 tables and 5 figures.)