Carboplatin, a new analogue of cisplatin, was administered into the serous cavity in nine primary lung cancer patients with malignant effusion, consisting of six malignant pleural effusions, two malignant pericardial effusions and one malignant ascites. Clinical effects, toxicities and pharmacokinetics were studied. The doses of carboplatin were 300 mg/m2 in seven patients, 200 mg/m2 in one patient and 1,100 mg/body in one patient. In seven evaluable patients, consisting of four non-small cell lung cancers and three small cell lung cancers, the response rate was 85.7% with 3 CR cases, 3 PR cases and 1 NR case. As toxicities, thrombocytopenia was observed in 57.1%, leukopenia in 57.1%, anemia in 71.4%, anorexia in 42.9%, nausea or vomiting in 28.6%, and low grade fever in 14.3%. However local pain, renal or liver dysfunction were not observed. The pharmacokinetics of free platinum concentration was analyzed with a two-compartment model (t1/2 beta = 18.60 hours) and 14.8% of total platinum remained free in effusion 24 hours after intracavitary administration. A high level of free platinum in effusion was maintained over a long period after carboplatin administration. This method was considered to be effective for the treatment of malignant effusion from the viewpoint of pharmacokinetics and less toxicity.