To reveal useful prognostic factors in cases of small-sized pulmonary adenocarcinoma, we conducted a histological and karyometric analysis of 116 small-sized pulmonary adenocarcinomas measuring less than 2 cm in maximum diameter and four specimens of atypical adenomatous hyperplasia (AAH). The small-sized pulmonary adenocarcinomas were classified by using criteria described previously [Noguchi M, Morikawa A, Kawasaki M, et al. Small adenocarcinoma of the lung. Histologic characteristics and prognosis. Lung Cancer 1995:75;2844-52]. There were 99 tumors of replacement-type adenocarcinoma, comprising 11 type A, localized bronchioloalveolar adenocarcinoma (LBAC); 6 type B, LBAC with alveolar collapse; and 82 type C, LBAC with foci of fibroblastic proliferation. The 17 remaining tumors were non-replacement-type adenocarcinomas. Among the potential prognostic factors examined, histological subtype was the most closely correlated with 5-year relapse-free survival rate. Furthermore, in patients with type C adenocarcinomas, a small fibroblastic proliferation (F) to fibrosis area (f) ratio (F-f ratio) (<10%) of the tumor and a small maximum nuclear diameter (Max ND; <13.50 microm) of tumor cells were closely associated with an excellent prognosis. Histological subtypes of type A and B adenocarcinomas, a small F-f ratio, and a small Max ND of type C adenocarcinomas were closely correlated with an excellent prognosis in small-sized adenocarcinoma.