The response and pharmacokinetics of cisplatin instilled into the pleural cavity were studied in 11 patients with malignant pleural effusion; 10 patients had primary lung cancer and one had breast cancer. All of them were adenocarcinoma histologically. In five of the 11 patients effusion disappeared and its cytology became negative for malignancy after four weeks. In the other six patients effusion was reduced and its cytology became negative for malignancy after four weeks. Toxicity was almost similar to that in systemic administration of cisplatin but a few patients had chest pain and fever possibly due to local irritation. The pharmacokinetics showed that a high concentration of cisplatin (free-form, 48.9 micrograms/ml) was maintained over a long period (free from (t 1/2) beta = 33.6 hours) in the pleural cavity. This was regarded as the reason for the high response to this therapy. The intrapleural instillation of cisplatin into the pleural cavity therefore seems to be an effective modality for malignant pleural effusion.