Dimensions of instruction and assessment were manipulated within a tertiary prevention to examine their effects on word and text reading development. In this article case studies of two children who demonstrated significant and sustained lack of adequate response to treatment are profiled. Results indicated that reducing group size, using formative assessment, and implementing mastery learning criteria produced positive trends on indicators of oral reading fluency. Students' reading outcomes provide evidence of the importance of timely data-based instructional adjustments. (Contains 1 table and 1 figure.)