Teacher professional development (PD) is seen as a promising intervention to improve teacher knowledge, instructional practice, and ultimately student learning. While research finds instances of significant program effects on teacher knowledge, little is known about how long these effects last. If teachers forget what is learned, the contribution of the intervention will be diminished. Using a large-scale data set, this study examines the sustainability of gains in teachers' content knowledge for teaching mathematics (CKT-M). Results show that there is a negative rate of change in CKT after teachers complete the training, suggesting that the average score gain from the program is lost in just 37 days. There is, however, variation in how quickly knowledge is lost, with teachers participating in summer programs losing more rapidly than those who attend programs that occur during school years. The implications of these findings on designing and evaluating PD programs are discussed.