This study used three different differential item functioning (DIF) procedures to examine the extent to which items in a mathematics performance assessment functioned differently for matched gender groups. In addition to examining the appropriateness of individual items in terms of DIF with respect to gender, an attempt was made to identify factors that may be related to DIF. The QUASAR Cognitive Assessment Instrument (QCAI) is designed to measure mathematical thinking and reasoning skills through open-ended questions. In this study, 33 polytomously scored QCAI items, from 4 test forms completed by 1,782 6th and 7th graders, were evaluated for gender-related DIF. Results suggest that DIF may not be serious for 31 of the 33 QCAI items. Explanations are suggested for the DIF of the remaining two items, one of which is of particular concern. Three figures and six tables present study findings. (Contains 29 references.) (SLD)