Fibronectin (FN) was investigated in 75 stroma from lung adenocarcinoma patients. Distribution of FN was classified into three patterns based on immunohistochemical evidence: dense, moderate, and sparse distribution. The incidence of the sparse pattern in both stage I and stage III disease was significantly greater in the infiltrating than in the noninfiltrating areas. The patients were divided into two groups, long-term survivors and short-term survivors. In the group with stage I disease, the incidence of the sparse pattern was significantly greater in the infiltrating than in the non-infiltrating areas. In both areas, the incidence of sparse pattern was greater in the short-term than in the long-term survivors. These data suggest that the disappearance of FN reflects the aggressiveness of the tumor and presumably plays an important role in the prognosis of adenocarcinoma of the lung.