Thirty-three patients with microscopically verified primary or metastatic malignant pleural effusion were studied: 7 had malignant mesothelioma and 26 metastatic pleural disease. The treatment was based on biochemical and clinical studies which show a synergy between cytosine-arabinoside (Ara-C) and cisplatin. These drugs were instilled in the pleural cavity at the dose of 100 mg for Ara-C and 100 mg/m2 for cisplatin. The cavity was drained after 4 h. If it was possible, the treatment was repeated weekly for 3 times and, after a 6-week rest, it could be started again with the same schedule. The overall response rate (complete plus partial remissions) was 74%. Toxicity was mild or moderate. We conclude that the combination of Ara-C and cisplatin is well tolerated and produces a high response rate in the treatment of malignant pleural effusions.