A group of fourth-grade students with disabilities participating in statewide mathematics assessments in the spring of 2001 (n=570) was given a new assessment focused on specifically modified state curricular standards defined for students with disabilities. A block of items from that assessment was also given to students taking the general mathematics assessment (n=1,944, including 235 students with disabilities), allowing the difficulty of all items to be estimated on a common scale using item response theory methodology. This study compared the characteristics of the two tests. Findings support the conclusion that a simpler test, based on the same curricular objectives, but providing better measurement of low-achieving students, could be developed and used for this special population. (Author/SLD)