Partial outlet obstruction of the rabbit urinary bladder results, initially, in a rapid increase in bladder mass and remodeling of the bladder wall. Previously, it was shown that this response was characterized by serosal growth (thickening) which was apparent after 1 day of obstruction, before any visible vascularization was observed. After 1 week of obstruction, significant microvessel formation was seen in the transition region between the detrusor smooth muscle and the thickening serosa; after 2 weeks the entire serosa was vascularized. In this study we investigated the effect of chronic (4 week) partial outlet obstruction on microvessel density and distribution in the bladder wall immunohistochemically using CD31 as a marker for vascular endothelium. Transverse sections of bladder wall were examined after 4 weeks of no surgery, sham surgery or partial obstruction. The microvessel density of the obstructed rabbit bladder mucosa and detrusor smooth muscle increased relative to augmentation of these compartments while new vessels appeared in the thickening serosa. Although vessel density did not change with obstruction a significant shift in mean vessel circumference to the left occurred indicating a significant increase in the number of microvessels and small vessels consistent with angiogenesis.