Brain-specific angiogenesis inhibitor 1 (BAI1) is a novel angioinhibitory factor. We examined BAI1 gene expression in 48 pulmonary adenocarcinoma specimens by RT-PCR. Vascular density (number and measurement area) was immunohistochemically evaluated in tumor specimens using anti-CD34 monoclonal antibody. BAI1 gene expression was detected in 38 of 48 pulmonary adenocarcinomas (79. 2%). Vascular number and measurement area were significantly decreased in the BAI1-positive pulmonary adenocarcinomas (19.3+/-4. 4/microm2 and 1.7+/-0.6%) as compared with those in the BAI1-negative carcinomas (75.5+/-42.7/microm2 and 5.5+/-1.5%) (Mann-Whitney U-test, p=0.0017). These results suggest that BAI1 expression may inhibit stromal vascularization in pulmonary adenocarcinoma.