OK-432 (picibanil) is a potent immunostimulant which has demonstrated benefit in the treatment of several types of malignancies, malignant ascites, and pleural effusion. We report a female patient with premenopausal breast cancer presenting with refractory malignant right pleural effusion. Shortly after OK-432 pleurodesis, bilateral reactive pleural effusion developed, soon followed by dyspnea. The pleural effusion was improved by bilateral pigtail catheter replacement, and no more massive pleural effusion was noted in the following 4 months. This complication was possibly related to immune response, but further experience and investigation is needed to prove it.