District leaders in a large urban school district in central Florida wanted to examine the efficacy of a new curriculum designed to enhance the word knowledge of grade 5 students so as to improve reading achievement. The new curriculum, called Word Knowledge Instruction (WKI), consists of 15-minute lessons 4 days a week for 20 weeks. The lessons address state standards and cover 20 prefixes and suffixes. Thirty-nine schools participated in the study, with 92 English language arts (ELA) teachers in high-poverty schools randomly assigned within schools either to use WKI or to continue to use their standard ELA curriculum. Classroom observations revealed that WKI was implemented as intended. WKI had a positive effect, equivalent to an increase of 9 percentile points, on students' ability to correctly extract and spell a base word from a derived word, one of the skills explicitly taught by WKI. WKI had no effect on two other related reading skills that were not directly taught by WKI (students' ability to select a nonword that best fits the grammatical context of a sentence or to use knowledge of word parts to infer meaning of new words) or on students' vocabulary or reading scores. These findings suggest that, although students learned what they were explicitly taught, the transferability to related but not directly taught skills might require more intense or longer duration instruction or additional professional development for teachers. [For the study snapshot, see ED611684. For the appendix, see ED611685.]