BACKGROUND Although there are many reported prognostic indicators for pulmonary adenocarcinoma, the clinicopathologic characteristics and prognostic factors of early stage adenocarcinoma have not been evaluated fully, except for several studies of nonmucinous and sclerosing bronchioloalveolar carcinoma. METHOD Two hundred thirty-six surgically resected small peripheral adenocarcinomas measuring 2 cm or less in greatest dimension were reviewed using a simple histologic classification of six types based on tumor growth patterns. RESULTS Type A (localized bronchioloalveolar carcinoma [LBAC]) (n = 14) revealed replacement growth of alveolar-lining epithelial cells with a relatively thin stroma. In type B (LBAC with foci of structural collapse of alveoli) (n = 14), fibrotic foci due to alveolar collapse were observed in tumors of LBAC. Type C (LBAC with foci of active fibroblastic proliferation) (n = 141) was the largest group in this study, and foci of active fibroblastic proliferation were evident. Type D (poorly differentiated adenocarcinoma), type E (tubular adenocarcinoma) and type F (papillary adenocarcinoma with a compressive growth pattern) (n = 61) showed compressive and expanding growth. Types A and B showed no lymph node metastasis and the most favorable prognosis (100% 5-year survival) of the six types. CONCLUSION Histologic types A and B are thought to be in situ peripheral adenocarcinoma, whereas type C appears to be an advanced stage of types A and B. Conversely, types D, E, and F are small advanced adenocarcinomas with a less favorable prognosis.