It is argued that mastery learning is one explanation for the documented differences in mathematics achievement between Japanese and American students. Given its emphasis on mastery at one stage before moving on to the next stage, and the potential accumulated benefits of this approach over time, mastery learning appears to be very similar to what occurs in Japanese classrooms. A meta-analysis of studies of elementary mathematics achievement supports the research findings of H. W. Stevenson and S. Y. Lee (1990). The overlapping confidence intervals of Japanese mathematics performance and mastery learning do not support a positive conclusion that mastery learning caused the differences, but they do indicate that there is a potential relationship. The role of mastery learning in mathematics deserves further exploration. An appendix contains a chart of the studies reviewed. (Contains 3 figures and 18 references.) (SLD)