Sixty-seven breast cancer patients with malignant pleural effusions underwent their first thoracentesis between 1980 and 1990 at 13 institutes in Kyoto and Shiga Prefectures. They received various intracavitary treatments including OK-432 and cultured effusion lymphocytes (n = 29, group A), OK-432 alone (n = 12), chemotherapeutic agents alone (n = 9), OK-432 and chemotherapy (n = 16) or other (n = 1, lentinan and tetracycline) therapy. Response rate and median survival time were 90%, 12 months for group A, respectively and 40% and 3 months for other treatments (group B, n = 38), resulting in significant differences. Multivariate analysis using Cox's proportional hazard model showed that the immunotherapy using cultured lymphocytes was the most important factor prolonging survival among several significant prognostic factors such as concomitant liver metastasis, disease-free period and laterality of effusion.