Purpose Cardiac CT scans for the assessment of coronary calcium scores include approximately 70% of the lung volume and may be useful for the quantitative assessment of emphysema. The reproducibility of lung density measures from cardiac CTs and their validity compared to lung density measures from full-lung scans is unknown. Methods and Methods The Multi-Ethnic Study of Atherosclerosis (MESA) performed paired cardiac CT scans for 6,814 participants at baseline and at follow-up. The MESA-Lung Study assessed lung density measures in the lung fields of these cardiac scans, counting voxels below -910 HU as moderate-to-severe emphysema-like lung regions. We evaluated: 1) the reproducibility of lung density measures among 120 randomly selected participants, 2) the comparability of measures acquired on electron-beam CT (EBT) and multidetector CT (MDCT) scanners among 10 participants; and 3) the validity of these measures compared to full-lung scans among 42 participants. Limits of agreement were determined using Bland-Altman approaches. Results Percent emphysema measures from paired cardiac scans were highly correlated (r=0.92-0.95) with mean difference of -0.05% (95% limits of agreement: -8.3, 8.4%). Measures from EBT and MDCT scanners were comparable (mean difference -0.9%; 95% limits of agreement: -5.1, 3.3%). Percent emphysema measures from MDCT cardiac and MDCT full-lung scans were highly correlated (r=0.93) and demonstrated reasonable agreement (mean difference 2.2%; 95% limits of agreement: -9.2, 13.8%). Conclusion While full-lung imaging is preferred for the quantification of emphysema, the lung imaging from paired cardiac CTs provided a reproducible and valid quantitative assessment of emphysema in a population-based sample.