An online professional development program offered a series of online training courses to K-12 teachers for gifted education in the US for three years. This study explores the factors influencing teachers' decision on drop out of the online courses. Non completing teachers participated in an online survey and phone interviews. In addition, Myths and Reality pre-test scores were compared between the group of teachers who successfully completed the courses and the group of teachers who did not complete them. As a result, unexpectedly, difficulties with technology was not found to be a deterrent to the completion of the online courses. The most significant influence on dropping a course was the stress of daily life and the lack of time to study due to increased responsibilities and commitment at the school and home. Another factor was dissatisfaction with the instructors. Particularly, the dropout teachers suggested the need to have a conflict resolution policy in place. Based on the analysis of the Myths and Reality pre-test, teachers who had more misconceptions of giftedness were more likely to drop out than those who possessed less misconceptions. Based on the findings of the study, effective strategies for reducing teachers' dropout rates in online professional development courses were suggested.