The purpose of this replication study was to examine the impact of the Teacher Study Group (TSG) professional development in vocabulary on first-grade teachers' knowledge of vocabulary instruction and observed teaching practice, and on students' vocabulary knowledge. Sixty-two schools from 16 districts in four states were randomly assigned to treatment and control conditions. A total of 182 first-grade teachers and their 1,811 students formed the sample. Treatment teachers received the TSG professional development from October to March. Teachers in the business-as-usual control condition received school/district professional development. A multilevel analysis was conducted to detect impacts. Significant impacts were found for teacher knowledge and observed teaching practice. No impacts were found at the student level.