The purpose of this study was to examine the effects of an after-school music program on music underachievers' musical achievement, social development and self-esteem. A true-experimental pretest-posttest design was used and included 14 hours of treatment time. The subjects (N = 66), fifth-grade students were randomly selected from the lowest quartile of scores on Colwell's Music Achievement Test (MAT), which was administered to all fifth-grade students (N = 494) in three Korean elementary schools. The treatment group (n = 33) experienced a movement-based after-school music program (MAMP); the control group (n = 33) did not receive the after-school music program. Measurements included sections of Colwell's Music Achievement Test (MAT), Kim's Social Development Scale (SDS), and Hare's Self-Esteem Scale (HSS). The researcher and music teachers of each school administered all measurements. Fourteen treatment lessons occurred over fourteen weeks. One-way analyses of covariance tests were used to test for post-test differences between groups. A significant difference was found in music achievement total scores of the MAT with the treatment group scoring higher scores than the control group. There were no significant differences for interval and meter discrimination tests of MAT. There were no significant differences between treatment and control groups in the post-test scores of the Social Development Scale (SDS) and the Self-Esteem Scale (HSS). However, for both tests, mean scores increased for the treatment group and decreased for the control group. Results from this study suggest that a movement-based after-school music program promotes music underachievers' musical growth and may also support music underachievers' social development and self-esteem.