Bronchiolitis obliterans organizing pneumonia (BOOP) and usual interstitial pneumonia (UIP; ie, cryptogenic fibrosing alveolitis of mural type, CFA) are clinically and histologically distinguishable interstitial lung diseases. Both contain intraluminal lesions of newly formed fibromyxoid connective tissue. In BOOP, the fibromyxoid lesions are susceptible to complete reversal, but in UIP they are supposed to participate in the remodeling of the interstitium. Our hypothesis was that capillarization of the intraluminal fibromyxoid lesions is more frequent in BOOP compared with UIP. In this study, we stained diagnostic thoracoscopic or open lung biopsy specimens of patients with BOOP (n = 9) and UIP (n = 10) with antibodies against human laminin, von Willebrand factor, and CD34 to reveal the microvasculature of intraluminal fibromyxoid lesions. Our results show that in BOOP there is abundant capillarization in the newly formed intraluminal fibromyxoid lesions often reminiscent of granulation tissue. The mean number of capillaries per area unit (mm2) was 107 +/- SD 74 in samples stained for laminin, 103 +/- SD 46 for von Willebrand factor, and 63 +/- SD 36 for CD34. In marked contrast, in UIP, the corresponding accounts were significantly lower, being 14 +/- SD 15 for laminin (P < .003), 11 +/- SD 14 for von Willebrand factor (P < .001) and 6 +/- SD 6 for CD34 (P < .001). The intraobserver (P < .001) and interobserver correlations (P < .002) were highly significant, showing that our results are reproducible. We conclude that the content and nature of the newly formed intraluminal connective tissue, for example, in the form of vascular growth factors, are different in BOOP and in UIP, and this partly leads to the different clinical course of these diseases.