As teacher professional development research includes more experimental designs, new challenges arise. We examine the threat of participant attrition as an example of the types of problems researchers face. Counter-intuitively, higher levels of recruitment effort were related to higher dropout rates among teachers. We also found that teachers left because of changes in teaching assignments, institutional challenges, and personal challenges. Finally, the majority of teachers in urban schools dropped out, while the majority of teachers in rural or suburban schools remained. We conclude with recommendations for future teacher learning experiments. (Contains 3 tables.)