OK-432 is an immunomodulator that has been reported to be efficacious as an injection therapy for cervical lymphomas and ranulas. We performed OK-432 injection therapy to treat a cystadenocarcinoma of the parotid gland in a 72-year-old man. The 50 x 46-mm tumor was located in the deep lobe of the gland. The tumor had compressed the glossopharyngeal, vagus, and hypoglossal nerves, causing neurally mediated syncope, hoarseness, dysphagia, and dysarthria. A concentration of 5 KE/2 ml of OK-432 was injected. Within 2 months, the cyst had disappeared; no recurrence was apparent during 59 months of follow-up. To the best of our knowledge, no previous report has described injection of OK-432 for malignant cystic disease. We describe the injection method, injection dose, and postinjection course in the hope that this information will prove useful for future applications against malignant cystic disease.