The Noguchi criteria are useful in assessing the prognosis of patients with small lung adenocarcinoma. Although there is a significant difference in prognosis between type A or B and type C, it is difficult in some cases to distinguish these types accurately by microscopy. In this study, we used immunohistochemistry to examine alpha-smooth muscle actin (alpha-SMA) produced by active fibroblasts in 25 pulmonary adenocarcinomas less than 2 cm in diameter. Eleven of type C (61%) showed positive staining for alpha-SMA, whereas no positive cases were seen in type A or B. The incidence of cancerous blood vessel and lymphatic invasion were significantly higher in alpha-SMA positive cases than in negative cases, and the positive cases showed poorer prognosis. These findings indicate that immunohistochemical detection of alpha-SMA is useful and essential for histological typing by the Noguchi criteria.