The purpose of the study was to measure students' reported test-taking effort and the relationship between reported effort and performance on the Trends in International Mathematics and Science Study (TIMSS) Advanced mathematics test. This was done in three countries participating in TIMSS Advanced 2008 (Sweden, Norway, and Slovenia), and the present study is the first to make a cross-national comparison of reported effort in the TIMSS context. Students in all three samples on average reported a rather low level of motivation to spend effort on the low-stakes TIMSS test and in all nations, there was a statistically significant relationship between reported effort and test performance. These patterns of results were similar for the three nations, although there were also differences. The Swedish students reported the lowest level of invested effort, and the relationship between reported effort and test performance was strongest for Sweden. Although the study has limitations, findings are potentially important and suggest that student effort and motivation needs continued monitoring in low-stakes assessment contexts such as TIMSS, Programme for International Student Assessment (PISA), or National Assessment of Educational Progress (NAEP).