Pulmonary neoplasms can deposit malignant cells in the pleural cavity by a variety of mechanisms, depending not only on histological type but also on clinical stage. We investigated the effect on survival rate of a finding of malignant cells in pleural lavage. We also investigated the effect of the presence of pleural metastasis on postoperative course of disease. Two hundred surgical patients at Hospital General Universitario in Valencia between 1 February 1990 and 30 March 1993 were studied. Two groups were formed. Group one: 150 patients with lung cancer, none of whom had had pleural effusion prior to suffering transthoracic puncture during the preoperative study. Patients treated with parallel chemotherapy or radiotherapy were excluded in order to circumvent false positives. Group two: 50 patients with no tumors who underwent thoracotomy for reasons other than lung cancer. All patients underwent pleural lavage with saline before and after lung exeresis. We found 26.6% (40/150) positive cytologies in the pre-and postoperative lavages in the first group. None were found in the control group.