The pathologic features of invasion such as stromal disruption and pleural/vascular involvement have been shown to be of prognostic value in adenocarcinoma. However, the relationship between the degree of invasion, histologic subtype of adenocarcinoma, and prognosis remains unclear. We retrospectively studied 380 peripheral adenocarcinomas of < or = 2.0 cm in diameter with regard to histology and clinical profiles. Their degree of invasive growth was classified into four grades as follows according to the structural deformity and its location in the adenocarcinoma lesion: Grade 0 had a pure bronchioloalveolar growth pattern and no evidence of stromal invasion. Grade 1 had stromal invasion in the area of bronchioloalveolar growth. Grade 2 had stromal invasion localized on the periphery of a fibrotic focus. Grade 3 had stromal invasion into the center of a fibrotic focus. The clinicopathological data were obtained from medical records. The distribution of the histologic grade of invasion was as follows: grade 0 in 85 tumors (22%), grade 1 in 37 (10%), grade 2 in 46 (12%), and grade 3 in 212 (56%). This histologic grade of invasion was closely related to other indicators of tumor spread. Vascular/lymphatic permeation was seen in none of grade 0, in 1 lesion each of grade 1 and grade 2, and 144 (68%) of grade 3. Lymph node metastasis was seen in 57 (27%) lesions of grade 3 but not in grades 0, 1, or 2. The 5-year disease-free survival rates were 100%, 100%, 100%, and 59.6% for tumors with grade 0, grade 1, grade 2, and grade 3 invasion, respectively. Tumors with grade 1 and grade 2 invasion, like tumors with grade 0 invasion (bronchioloalveolar carcinoma), showed an excellent prognosis. Therefore, tumors with grade 1 and grade 2 invasion could be considered "minimally invasive" or "early" adenocarcinomas.