Lungs of newborn and of 7- and 30-day-old pigs were fixed at total capacity by a standardized procedure. Stereologic morphometry was applied to random ultrathin sections of parenchyma. Relative volume density, absolute volume, and capillary surface coverage of intravascular macrophages as well as parameters of relative morphometric diffusion capacity of lungs were assessed. Intravascular macrophages occupied up to 25% of capillary volume in 30-day-old pigs compared with 6% in newborn pigs; this was a 14-fold increase of absolute volume. In 30-day-old pigs, 16% of capillary surface was covered with closely adherent intravascular macrophages in contrast to only 2% in newborn pigs. Determinators of morphometric diffusion capacity were similar in all pigs. Greater thickness of the arithmetic and harmonic mean air-blood tissue barrier in 30-day-old pigs (with included thickness of intravascular macrophages) compared with air-blood tissue barriers (without intravascular macrophages) was not statistically significant. The relative difference of arithmetic mean was larger than the difference of harmonic mean, and reflected preferential adherence of intravascular macrophages to the thick portion of the air-blood tissue barrier. Our findings confirm morphologic observations of perinatal colonization of the porcine lung by monocytes that replicate and differentiate into large, highly phagocytic, resident, intravascular macrophages. Preferential adherence to sites where basal laminae of capillary endothelium and alveolar epithelium were separated by interstitium may explain the minimal morphometric impact of intravascular macrophages on the air-blood tissue barrier thickness. This location would be expected to have less effect on gas diffusion than location of intravascular macrophages on the thin side of the air-blood barrier.