Previous work has demonstrated the importance of the Protein Kinase C (PKC) signal transduction system in regulating the growth rate of malignant gliomas in vitro. Tamoxifen inhibits PKC in a minority of malignant gliomas within the micromolar concentration range in vitro, a property distinct from its estrogen receptor blockade effect. Tamoxifen was administered orally in very high dosages to 11 patients (9 males:2 females, age range 26-73, mean 45 years) with malignant gliomas (anaplastic astrocytoma or glioblastoma multiforme) who had failed treatment with external beam radiation therapy (and additional chemotherapy in 2). The dosage administered was estimated to be that necessary to achieve tissue concentrations within the low micromolar range, shown necessary to inhibit PKC in these tumors in vitro, and is approximately 5 times that used for standard antiestrogen therapy. Tumor reduction on radiographic images (MRI and PET [18FdG uptake]) with clinical improvement occurred in 3 patients; halting of tumor progression clinically and radiographically occurred in an additional patient. Of the remaining seven patients, three patients had marked and rapid progression of their disease despite treatment (dead after 3, 4, and 6 months respectively). Complications of treatment included a deep venous thrombosis requiring anticoagulation in one patient, nausea in one patient, and "hot-flashes" in a third patient. Tumor biopsy and measurement of tamoxifen and its active metabolite within the tumor of one patient (non-responder) showed levels within the middle of the in vitro therapeutic range. Follow-up of alive patients ranges from 4-18 months (mean 10 months). These encouraging preliminary results in a minority of these patients suggests some potential for this type of therapy.(ABSTRACT TRUNCATED AT 250 WORDS)