Research shows that children's reading proficiency levels in the early grades positively correlate with students' future academic achievement. This study provides the first-ever analysis of reading achievement trajectories for a cohort of students in grades 3 to 5 in 2014-17 in Hawaii schools. Hawaii serves a diverse student population whose characteristics differ in ways often overlooked by standard US racial and ethnic classifications. Our analysis shows that Native Hawaiian and Pacific Islander students not only start at a lower reading proficiency than their peers in Grade 3, but the achievement gap widens as they move from Grade 3 to Grade 5. Moreover, we find a strong association between students' third-grade performance and reading achievement growth rate above and beyond all other factors in our longitudinal model. The difference in performance patterns between student subgroups across the elementary grades can serve as an accurate baseline for yearly monitoring. In light of our findings, we discuss implications for policy and practice.