Angiogenesis and vascular remodeling are features of many chronic inflammatory diseases. When diseases evolve slowly, the accompanying changes in the microvasculature would seem to be similarly gradual. Here we report that the rate of endothelial cell proliferation and the size of blood vessels increases rapidly after the onset of an infection that leads to chronic inflammatory airway disease. In C3H mice inoculated with Mycoplasma pulmonis, the tracheal microvasculature, made visible by perfusion of Lycopersicon esculentum lectin, rapidly enlarged from 4 to 7 days after infection and then plateaued. Diameters of arterioles, capillaries, and venules increased on average 148, 214, and 74%, respectively. Endothelial cell proliferation, measured by bromodeoxyuridine (BrdU) labeling, peaked at 5 days (18 times the pathogen-free value), declined sharply until day 9, but remained at approximately 3 times the pathogen-free value for at least 28 days. Remodeled capillaries and venules were sites of focal plasma leakage and extensive leukocyte adherence. Most systemic manifestations of the infection occurred well after the peak of endothelial proliferation, and the humoral immune response to M. pulmonis was among the latest, increasing after 14 days. These data show that endothelial cell proliferation and microvascular remodeling occur at an early stage of chronic airway disease and suggest that the vascular changes precede widespread tissue remodeling.