We examine the effects of a year-long national-scale professional development (PD) program on mathematics instructional quality. In contrast to previous studies examining the effects of this program on instruction by comparing before and after participation or participants and non-participants, we examine whether instructional quality changed during the program. More specifically, we conduct an analysis of 174 video-recorded mathematics lessons given by 52 teachers during their year of participation. Contrary to previous studies, the results demonstrate that the instructional quality did not improve over the course of the PD. We suggest that the explanations for the diverging results concern how, when, and to what extent instructional quality changes in PD programs. Specifically, we discuss how the explanations may illuminate the significance of PD duration for PD effects, and how these effects may be mediated by features concerning the PD content and the scale at which the program is implemented.