This study compared the validity of progress monitoring slope of nonsense word fluency (NWF) and word identification fluency (WIF) with early first-grade readers. Students (N = 80) considered to be at risk for reading difficulty were monitored with NWF and WIF on a 1-2 week basis across 11 weeks. Reading skills at the end of first grade were assessed using measures of passage reading fluency, real and pseudoword reading efficiency, and basic comprehension. Latent growth models indicated that although slope on both measures significantly predicted year-end reading skills, models including WIF accounted for more variance in spring reading skills than NWF, and WIF slope was more strongly associated with reading outcomes than NWF slope. Analyses of student growth plots suggested that WIF slope was more positively associated with later reading skills and discriminated more clearly between students according to successful or unsuccessful year-end reading outcomes. Although both measures may be used to monitor reading growth of at-risk students in early first grade, WIF may provide a clearer index of reading growth. Implications for data-based decision-making are discussed.