Attitudes toward mathematics are very important in the achievement and participation of students in mathematics. Declining national test scores in mathematics and dislike of mathematics have increased recognition of the problem of student attitudes. The purposes of this study were to develop an instrument to measure students' attitudes toward mathematics (ATMI) and to find the underlying dimensions that comprise the ATMI. The sample consisted of 544 students taking mathematics at a private bilingual preparatory school in Mexico City. Students were asked to indicate their degree of agreement with each statement, from strongly disagree to strongly agree. Variables considered were value, anxiety, motivation, confidence, enjoyment, and adults' perspectives. The alpha reliability coefficient for the whole instrument was .96. After dropping the nine weakest items, the reliability increased to .97. A principal components factor analysis with a varimax (orthogonal) rotation revealed students' sense of security, value of mathematics, motivation, and enjoyment of mathematics. The ATMI psychometric analysis revealed sound properties and can therefore be used by researchers and practitioners to measure students' attitudes toward mathematics. Contains 24 references. (Author/PVD)