A prospective randomized study to compare the effectiveness of pleurodesis by two new sclerosing agents: OK-432 and mitomycin C were conducted in 53 patients with malignant pleural effusion caused by lung cancer. None of the patients received concomitant systemic chemotherapy or radiation therapy during the study. After complete drainage of pleural fluid, the patients were allocated randomly to receive 10 Klinische Einheit units of OK-432 or 8 mg of mitomycin C by intrapleural injection at weekly intervals. The treatment was terminated if the pleural effusion disappeared or the patients had received four consecutive procedures. There were 26 patients who received pleurodesis with OK-432 and 27, with mitomycin C. Patient characteristics in the two treatment groups (age, sex, histologic type, performance status, and prior treatment before pleurodesis) were compatible. These results showed that pleurodesis with OK-432 achieved a higher complete response rate (73%) than that of mitomycin C (41%). The rates of objective treatment response (complete response plus partial response) were comparable in both groups (88% for OK-432 and 67% for mitomycin C). The average number of intrapleural injections needed to achieve complete response was fewer in the OK-432 group (1.9 +/- 0.9) than in mitomycin C group (2.8 +/- 0.9). There was no significant difference in the median survival of the patients who received pleurodesis with OK-432 (5.8 months) or mitomycin C (5.1 months). However, the effusion-free period in the OK-432 group was significantly longer than that in the mitomycin C group (7.0 months versus 1.5 months). Patients who underwent OK-432 pleurodesis had a higher complication rate (80%) than did those in the mitomycin C group (30%). Transient febrile reaction was the most common reaction encountered. The immunologic study in OK-432 group showed an increase in peripheral leukocyte count and decrease in the OKT4/OKT8 ratio. The mitomycin C group had a mild reduction in peripheral blood leukocyte count and no significant change in the OKT4/OKT8 ratio. It was concluded that pleurodesis with OK-432 is an effective alternative treatment for malignant effusion in patients with lung cancer.