The present study examines the extent to which participation in a 14-week professional development course designed to improve teacher-child interactions in the classroom moderated the relation between teacher-reported job stress and gains in observed teacher-child interaction quality from the beginning to the end of the intervention. Participants were preschool teachers (N = 427; [mean] age = 42) with an average of 11 years of experience teaching. Teachers reported how intensely they experienced different sources of stress at pre-test only (i.e., prior to being randomized into the treatment condition [course or control]). Teacher-child interactions were measured through classroom observations at pre and post intervention. Results demonstrated that control teachers reporting higher professional investment stress showed fewer gains in observed emotional support relative to control teachers experiencing less professional investment stress. These findings were not evident for teachers in the course condition. Interestingly, teachers with higher professional investment stress showed fewer gains in instructional support in the control condition and greater gains in the course condition, relative to teachers in their respective treatment groups who reported lower levels of professional investment stress. Findings suggest that participation in the professional development intervention had a buffering effect on the negative association between professional investment stress and emotional support. With regard to instructional support, it is possible that teachers' heightened awareness and anxiety over their need to develop professionally may have made them more responsive to an intervention designed to improve practice. (author abstract)