This causal-comparative study examined the relationship between enrollment/non-enrollment in music programs of differing content or quality and mathematical achievement among 739 secondary (grades 8-12) students from four different Maryland counties. The students, both female and male, were divided into sample groups by their participation in a general music or music performance class during their eighth-grade school year. The students were further divided into three groups based on their class's score at the district music festivals resulting in five music groups (nonmusic, general music, music performance with a score of I [the highest score], music performance with a score of II, and music performance with a score of III). The researcher used the retrospective causal-comparative approach to examine the relationship between music instruction and mathematical achievement. The results showed that students who participated in music-performance programs (instrumental or choral) scored significantly higher on mathematical measures (Maryland School Assessment [MSA] and High School Assessment Algebra/Data Analysis [HSA]) although the effect size was small. The quality of the music performance programs did not have a significant effect. Findings indicated that students who chose to participate in music performance programs were more skilled in mathematics prior to their enrollment in music programs. ANCOVA revealed significantly higher gains in mathematics achievement for the music performance group with a score of III as compared to both non-music students (p<0.05) and general-music students (p<0.05). However, the higher rated performance groups did not significantly differ from the nonmusic and general-music students. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]