This study investigated whether two different versions of an online professional development course produced different impacts on the intended outcomes of the course. Variations of an online course for middle school algebra teachers were created for two experimental conditions. One was an actively facilitated course with asynchronous peer interactions among participants. The second was a self-paced condition, in which neither active facilitation nor peer interactions were available. Both conditions showed significant impact on teachers' mathematical understanding, pedagogical beliefs, and instructional practices. Surprisingly, the positive outcomes were comparable for both conditions. Further research is needed to determine whether this finding is limited to self-selected teachers, the specifics of this online course, or other factors that limit generalizability. (Contains 6 tables.)