In this study, we compared 2- and 3-D sizes of identical pulmonary alveoli as represented in OCT images versus micro-CT images. A correlation analysis showed that OCT provides highly representative measures of relative alveolar cross-sectional area (, ), perimeter (, ), volume (, ), and surface area (, ). However, when OCT measurements were plotted as a function of micro-CT measurements, the slope of the linear fit was smaller than 1 for all size parameters, equal to 0.78 for cross-sectional area, 0.88 for perimeter, 0.62 for volume, and 0.78 for surface area, indicating that OCT systematically underestimated the absolute alveolar sizes compared to micro-CT. Consequently, OCT did not allow the prediction of absolute alveolar sizes as measured by micro-CT. This is visualized in Fig. 3 and confirmed by a linear regression analysis showing that the slope in Fig. 3 was significantly different from 1, with the -intercept significantly different from 0 for cross-sectional area (, , ) and volume (, , ). The relationship was not significant for perimeter (, , ) or surface area (, , ).