Pleural lavage cytology immediately after thoracotomy was performed in 467 patients with lung cancer who had little or no pleural effusion. Forty-two patients (9.0%) had positive results. The positivity of pleural lavage cytology was significantly related to the degree of pleural extension of the tumor, microscopic pleural dissemination, cytologic results of minimal pleural effusion, pathologic stage, presence of lymphatic permeation or vascular invasion, and cell type (adenocarcinoma was predominant). The 3-year survival of the patients having negative and positive results of cytology were 68.7% and 22.9%, respectively. The prognosis of the group with positive results was as poor as that of patients with stage IIIB or IV disease. Pleural lavage cytology is an important prognostic factor that indicates microscopic exfoliation of cancer cells into the pleural cavity, that is, subclinical malignant pleural effusion.