Teaching breast self-care as breast changes occur in the adolescent girl can influence positive behaviors such as performing breast self-examinations and seeking regular professional breast examinations. Health promotion behaviors often are taught in high school, but little research has been performed on teaching breast health in a high school setting. Therefore, this descriptive study aimed to identify the beliefs, knowledge, and practices of breast self-examination in adolescent girls. A pre- and posttest design was used to determine whether a teaching program would change beliefs, knowledge, and practices of breast self-examination. The results of this study demonstrate that a one-time intervention can be successful in increasing breast self-examination practice and the knowledge of breast self-examination and cancer in adolescents. Before the intervention, approximately 66% of students said they never examined their breasts, and only 3% performed breast self-examination monthly. In 1 month after teaching, these percentages changed appreciably, with 32% of students reporting that they never practiced breast self-examination and 23% reporting that they practiced breast self-examination monthly.