This study investigates empirically the mechanisms behind the increasing grade point averages in Swedish upper secondary schools. Four hypotheses are presented as plausible explanations; improved student achievements, student selection effects, strategic behaviour in course choices, and lowering of grading standards. The analysis is based on extensive data, and focuses on grades and test scores from upper secondary school graduates over a 6-year period. The result shows that the increase in grade point averages cannot be explained by better achievements, selection effects or course choices, which means that standards have been lowered, which is interpreted here as grade inflation. The grade inflation is most likely an effect of the leniency in the grading system in combination with pressure for high grading, related to the upper secondary school grades' function as an instrument for selection to higher education.