The purpose of the present study was to investigate the reading growth of students with severe reading difficulties. The subjects in the four-year longitudinal data set were school-identified students with learning disabilities (LD) from a single school district in a large midwestern city. Since a single indicator was collected at each measurement period, the simplex structure was applied. The identified model in the present study was a perfect simplex model. The results also showed that reading is very stable over time, based on relative standings. Even among students with severe reading difficulties, the scores of students with higher reading skills continued to be higher across grades. The identified simplex structure in reading growth also showed increasing within-grade-level variability with higher grades. Even among the students with severe reading difficulties, the reading levels become more heterogeneous across grade levels. Implication of educating students with learning disabilities were also discussed.