This study used a statewide longitudinal sample to examine mathematics achievement gaps and growth in students with and without disabilities and to examine the impact of different methods of determining disability group membership on achievement gaps and growth. When disability status was determined on the basis of special education placement each year, the achievement gap was larger across grades than when the subgroup of students with disabilities (SWDs) was defined more broadly, including students who had exited special education or who were in special education anytime between Grades 3 and 7. Regardless of the identification criteria, the SWD subgroup showed lower average achievement and slower growth than students without disabilities. The results suggest that the present way of identifying the SWD subgroup in reporting achievement outcomes may be biased and that even students who exit special education continue to be at risk for lower mathematics achievement.