Despite seemingly radical surgery many patients operated on for bronchial carcinoma will die from their disease. Some patients might benefit from postoperative treatment and a prognostic factor that could identify those with an increased risk for tumor relapse would be of great clinical importance. One possible such factor is the occurrence of malignant cells in pleural lavage performed at operation. To test this hypothesis 224 consecutive patients who had been operated on due to verified or strongly suspected bronchial carcinoma, preoperatively staged as stage I or II, were investigated. After opening the thorax and before manipulation or palpation of the lungs, 300 ml of physiological saline solution was installed into the pleura. After excluding patients who were not radically operated, there remained 138 patients with histologically confirmed lung cancer (carcinoids excluded) and 12.3% showed tumour cells in the washings. Two of 18 patients with metastatic lung disease ( 11%) and one of ten patients with carcinoid tumor also showed malignant cells in the lavage. The patients with lung cancer have been followed for 3 years or until death. After three years 60.2% of those without malignant cells in the pleural lavage were still alive, while this figure was 41.2% in the other group. The difference was not statistically significant. Other factors, such as spread to local lymph nodes, size of tumor, etc. were related to the occurrence of malignant cells in the pleura, and these factors were also better prognostic ones. We conclude that the clinical use of pleural lavage cytology is limited.