This study examined gender-related differential item functioning (DIF) using a mathematics performance assessment, the QUASAR Cognitive Assessment Instrument (QCAI), administered to middle school students. The QCAI was developed for the Quantitative Understanding: Amplifying Student Achievement and Reading (QUASAR) project, which focuses on reasoning, problem solving, and communication. By including two QCAI administration occasions, it was possible to examine the stability of gender-related DIF over time. On one occasion, 1,947 students took 4 forms of the QCAI, and on the other 1,999 students took the 4 forms. Student responses were rated by middle school mathematics teachers trained in the scoring procedure. Results indicated that 4 of the 36 tasks favored females and 2 favored males with respect to uniform DIF. The tasks that favored females were set in a real-world context, suggesting that some of the features that have been associated with gender-related DIF in the past may not hold true for performance assessments. For example, male students may have been disadvantaged in providing explanations for their answers in this performance assessment. (Contains 4 figures, 13 tables, and 43 references.) (SLD)