We report here the presence of subepithelial myofibroblasts in pure bronchioloalveolar carcinoma and a subset of invasive lung adenocarcinoma. The subepithelial myofibroblasts we describe were observed in a peculiar location beneath the cancer cells in the alveolar septa. Immunohistochemically, they were positive for alpha-smooth muscle actin and calponin, but negative for desmin and h-caldesmon. To gain insight into their biological significance, we examined 116 surgically resected lung adenocarcinomas. The resected tumors included 13 bronchioloalveolar carcinomas, 20 mixed type adenocarcinomas with bronchioloalveolar carcinoma components, 57 papillary adenocarcinomas, 22 solid adenocarcinomas with mucin, and 4 acinar adenocarcinomas. All specimens were immunostained for alpha-smooth muscle actin to visualize the myofibroblasts. In all of the pure bronchioloalveolar carcinomas observed, the subepithelial myofibroblasts were completely preserved adjacent to the cancer cells. In mixed adenocarcinomas with bronchioloalveolar carcinoma components, subepithelial myofibroblasts were present in the bronchioloalveolar carcinoma components, but scanty in the invasive areas, where stromal myofibroblasts emerged between the cancer cell nests. Subepithelial myofibroblasts were retained, however, in the invasive areas of a subset of invasive adenocarcinomas. Survival analysis showed that the retention of subepithelial myofibroblasts in these invasive tumors was associated with low rates of lymphatic and vascular invasion, a low rate of lymph node involvement, and an excellent patient survival. These results suggest that subepithelial myofibroblasts increase in bronchioloalveolar carcinomas, but are gradually replaced by typical stromal myofibroblasts during progression into invasive cancer. A subset of invasive adenocarcinomas retains subepithelial myofibroblasts. Analysis of subepithelial myofibroblasts may be helpful in identifying a subset of lung adenocarcinoma with excellent prognosis.