The histologic prognostic factors of pulmonary adenocarcinomas of the lung less than 2 cm in diameter were analyzed in 75 patients who had undergone surgical resection. The pathologic stage, lymph node involvement, and pleural involvement were found to be the major determinants of prognosis (P less than 0.01). In addition, other single factors, such as tumor differentiation (P less than 0.01), vascular invasion (P less than 0.01), the degree of collagenization in the fibrotic focus (P less than 0.01), the standard deviation (SD) of nuclear areas (P less than 0.05), and mitotic index (P less than 0.05) correlated significantly with prognosis by the log-rank test on the Kaplan-Meier survival curves of these factors. Patients with dense infiltration of "T-zone histiocytes" survived significantly longer than those with less infiltration (P less than 0.05). Cox's proportional hazard general linear model analysis showed the importance of factors, such as lymph node or pleural involvement and the SD of nuclear area, when the pathologic stage was excluded, and of the mitotic index when all four factors were excluded to emphasize the cellular characteristics. It is possible to predict the postoperative prognosis of patients with small pulmonary adenocarcinoma more precisely by combination of the above histopathologic factors.