We build on research connecting teaching assignments to teacher effectiveness by examining the characteristics of secondary grades mathematics teachers with more course preparations--both the unique number of courses taught and courses never taught before--and by assessing whether additional course preparations predict teacher value-added. Regarding teacher characteristics, we find a mixed pattern of results. Both better and less well-credentialed teachers have more course preparations. Teachers with higher lagged value-added are assigned to fewer course preparations. Regarding teacher effectiveness, results from teacher fixed-effect models show that additional course preparations are negatively and significantly associated with value-added estimates. Further analyses indicate that teachers' number of new course preparations fully explain the negative results for total number of course preparations. That is, when teachers have to plan for, learn the content of, and teach a course for the first time, it adversely influences their effectiveness. Our results are slightly smaller than findings from recent teaching assignment studies but take on practical significance given the frequency with which teachers have a new course preparation.