PURPOSE: The Seattle Angina Questionnaire (SAQ) is widely used to measure disease-specific health status in coronary artery disease, but its psychometric properties have not been evaluated using multidimensional item response theory (MIRT). We evaluated the SAQ's dimensional structure, measurement precision, differential item functioning (DIF), and short form performance in a population-based cohort. METHOD: SAQ responses collected within two weeks of coronary angiography from 6,665 patients in the Alberta Provincial Project for Outcome Assessment in Coronary Heart Disease (APPROACH) registry were analyzed using a confirmatory five-factor multidimensional graded response model. Item fit, local dependence, and conditional standard errors were evaluated. DIF by sex and age was examined using a two-step anchored multigroup approach. Short forms were derived based on discrimination, test information, and DIF magnitude. RESULT: The confirmatory five-factor model demonstrated good global fit (RMSEA = 0.031, CFI = 0.987, TLI = 0.982), supporting three distinct Physical Limitation dimensions: Basic, Moderate, and Strenuous. Empirical reliability ranged from 0.748 (Basic Physical Limitation) to 0.900 (Moderate Physical Limitation), with precision sufficient for individual-level interpretation in Moderate and Strenuous Physical Limitation. Moderate sex-related DIF was noted on Basic and Moderate Physical Limitation items, with females scoring lower than males. Age-related DIF was negligible to small. The 14-item short form preserved all five factors with theta correlations of 0.964 with the 18-item version. CONCLUSION: MIRT revealed three distinct Physical Limitation dimensions, enabling targeted functional monitoring. Precision gradients and sex-related DIF inform interpretation, and the 14-item short form is supported as the preferred version. The Seattle Angina Questionnaire (SAQ) is one of the most widely used tools for measuring how coronary artery disease affects patients' daily life, symptoms, and quality of life. Despite decades of widespread use, its measurement properties have never been examined using modern statistical methods. This study used a statistical approach, multidimensional item response theory, to examine how well the SAQ measures what it purports to capture. We analyzed responses from over 6,600 Canadian patients who had recently undergone a heart procedure. We found that physical functioning in heart disease patients is better understood as three separate dimensions: basic, moderate, and strenuous activities, rather than one combined score, though one of these (moderate activities) overlapped considerably with the others statistically. We also found that the questionnaire measures some health areas more precisely than others, and that women tend to score lower than men on physical activity items, even when their actual health is the same, which could lead to underestimating how heart disease affects women. Finally, we showed that a shorter 14-question version of the SAQ captures all five health areas with very little loss of information, making it practical for both routine clinical use and large research studies, and supporting more accurate and equitable use of the SAQ across different patient groups.