Mixed-aged (MA) classes are a common phenomenon around the world. In Sweden, these types of classes increased rapidly during the 1980s and 1990s, despite the fact that existing empirical support for MA classes is weak. In this paper, the effect of attending an MA class during grades 4-6 on students' cognitive skills is estimated. Using a unique survey with information on students, parents, and teachers, it is possible to control for many factors that could otherwise bias the results. A negative effect on short-run cognitive skills, as measured by grade 6 cognitive tests, was found. This effect is relatively large--almost 5 percentile points--and robust to a rigorous sensitivity analysis. On grade 9 credits the effect is still negative but smaller in size and not statistically significant. (Contains 1 figure, 10 tables, and 20 footnotes.)