The applicability of a standardized performance measurement procedure is critically investigated, in order to assess reading growth for students with severe reading difficulties in a longitudinal design. To establish an appropriate growth model, several approaches are reviewed. Among them, a simplex model is chosen and tested with various constraints. Participants in this 4-year longitudinal study were 64 elementary school students (29 boys and 15 girls) with reading disabilities. The results show that reading performance is stable over time based on relative standing, with a constant growth. The theoretical and practical implications of reading growth for students with reading disabilities under a perfect simplex model are examined and discussed. Includes five tables and one figure. (Contains 44 references.) (Author/SLD)