This study investigates the relationship between children's subitizing activity and their construction of arithmetic units. In particular, the study hypothesizes a positive association between children's construction of subitized units and their construction of arithmetic units, and hypothesizes that children who can subitize larger units, such as five items, in kindergarten, are more likely to construct arithmetic units by the beginning of first grade. Data for this study were drawn from 3,660 children surveyed at the beginning of kindergarten in 2014, 2015, and 2016, and at the beginning of first grade in the following year. The children are from a single school district in the southwest United States. Logistic regression was used to model the likelihood of constructing arithmetic units based on children's earlier construction of subitized units. Findings provide evidence of a positive relationship between children's construction of subitized units and arithmetic units, and, on average, children who have constructed subitized units at the beginning of kindergarten are more likely to construct arithmetic units by the beginning of first grade. Based on the findings, theoretical and instructional implications are discussed.