This study examined the validity of Nonsense Word Fluency as an index of beginning reading proficiency for students in kindergarten through second grade. Validity evidence for Nonsense Word Fluency is addressed in the context of research-based instructional practices implemented on a large scale. Technical adequacy data are presented for all students in participating schools, and separately for English learners and native English speakers. Five cohorts of students participated, with each cohort representing approximately 2,400 students. Results support the use of Nonsense Word Fluency in the early grades to screen students for reading problems and predict early reading proficiency. The use of this measure in reading reform is discussed as well as implications for school psychologists. (Contains 4 tables.)