The Third International Mathematics and Science Study- Repeat 1998/1999 (TIMSS) assessed the mathematics and science achievement of students in their second year of high school. In addition to achievement tests, extensive information was collected from students and teachers. Of the 38 countries that took part, the overall results in mathematics of eleven countries, including Australia were not statistically different. Following the TIMSS research model, this article examines the association between three levels of curriculum, the intended curriculum, the implemented and the attained curriculum. In particular, this paper compares the achievement in four algebra topics for those students whose teachers reported that the students had not been taught this topic, yet, with those who had been taught this topic for more than 5 hours. The results indicated that across the 10 countries, in some instances, there were significant differences in achievement between those students whose teachers reported that they had taught the topic and those who did not. However, it was not true of all algebra topics. Examples of items in each topic are presented and discussed to illustrate where the differences lie.