Abstract The scores of 1,700 Japanese and 9,582 Illinois high school students on the High School Mathematics Test (containing 60 items on algebra, geometry, modern mathematics, data interpretation, and probability with an internal consistency reliability of .87) were regressed on background questionnaire measures of several factors in learning. Quantity of instruction and motivational variables emerge as the stronger statistically controlled correlates of mathematics achievement in both Japan and Illinois. In addition, older students did better than younger, and males outscored females in Japan. The two-standard-deviation achievement advantage of Japanese over Illinois students at three age levels dwarfs the differences within countries and may be attributable to unmeasured extramural factors as well as to superior quantity and quality of instruction in Japan.