As Taiwanese students progress from elementary to junior high school, there is a remarkable increase in the inequality of achievement in mathematics. This increase is of a magnitude not seen in other countries. Findings show that the widening-gap phenomenon is accompanied by an exceptional increase in the percentage of students reaching the advanced international benchmark and a significant growth in the influence of family background on student performance. In Taiwan, the widening-gap phenomenon commonly manifests among students of different school entry cohorts, genders, and residential areas; the phenomenon occurs mainly within classrooms. Taiwanese students from more favorable family backgrounds are seen to improve significantly more in mathematics as they progress through the school grades, and they experience a less significant widening-gap phenomenon among themselves. This study explains the widening-gap phenomenon in relation to a national senior high school entrance examination administered to nearly all Taiwanese ninth graders.