This study used data from the U.S. state of Michigan schools participating in the "Reading First" program to examine the stability of students' disability status in Grades 1 to 3. Reading First aimed to improve at-risk students' reading comprehension through research-based instruction. We analyzed how changes in students' disability status were related to reading comprehension growth. The sample consisted of n = 10,196 students from 191 schools who took the Iowa Test of Basic Skills (ITBS) Reading Comprehension subtest. Data were collected from 2003 through 2008. We observed considerable changes in disability status for students with speech-language disorders or a specific learning disability (n = 1,193). These students had lower achievement levels and made less progress than students without disabilities. Overall, achievement gains did not differ between students with different stability and change patterns in their disability status. However, students changing from speech-language disorders to a specific learning disability demonstrated the smallest achievement gains.