In preclinical studies, selenite had single agent activity and radiosensitized tumors in vivo. Here we report results from a Phase 1 trial in 15 patients with metastatic cancer treated with selenite (5.5 to 49.5 mg) orally as a single dose 2 hours before each radiation therapy (RT) treatment. Patients received RT regimens that were standard of care. The primary objective of the study was to assess the safety of this combination therapy. Secondary objectives included measurement of pharmacokinetics (PK) and evaluation of efficacy. Endpoints included assessment of PK, toxicity, tumor response, and pain before and after treatment. The half-life of selenite was 18.5 hours. There were no adverse events attributable to selenite until the 33 mg dose level, at which the primary toxicities were grade 1 GI side effects. One patient treated with 49.5 mg had grade 2 GI toxicity. Although this was not a DLT, it was felt that the highest acceptable dose in this patient population was 33 mg. Most patients had stabilization of disease within the RT fields, with some demonstrating objective evidence of tumor regression. Most patients had a marked improvement in pain and seven out of nine patients with prostate cancer had a decrease in PSA ranging from 11-78%. Doses up to 33 mg selenite were well tolerated in combination with RT. A randomized, well controlled study is needed at the 33 mg dose level to determine if selenite results in clinically meaningful improvements in the response to palliative RT.