Reolysin is reovirus serotype 3-Dearing strain, a double-stranded replication-competent RNA non-enveloped icosahedral virus. It induces cytopathic and anti-cancer effects in cells with an activated ras pathway due to inhibition of the dsRNA-activated protein kinase.This was a single center dose escalation trial of Reolysin administered intravenously every 4 weeks in doses ranging from 1 x 10(8) to 3 x 10(10) tissue culture infective dose (TCID)(50). Serum for neutralizing antibody, and serum, stool, saliva, and urine for viral shedding were collected. Tumor samples were analyzed for activating mutations in the ras and braf oncogenes.Eighteen patients received 27 doses of Reolysin in 6 dose cohorts accomplishing a 300 fold dose escalation without a protocol-defined dose limiting toxicity. Drug related grade 2 toxicities included fatigue and fever (1 patient each). All patients developed neutralizing antibody during the course of the study. Viral shedding was observed in 6 patients. One patient with anthracycline and taxane refractory breast cancer experienced a partial response (PR) and her tumor had a ras G12A mutation. Biopsy from her chest wall mass showed evidence of necrosis and viral replication by electron microscopy. Overall clinical benefit (1 PR + 7 stable disease) rate was 45%, and appeared higher in patients with viral shedding (67%) than those without (33%).Reolysin administered monthly as a one-hour infusion is safe and well-tolerated even in multiple doses. Reolysin has anti-tumor activity as a single agent warranting further evaluation, including in combination with chemotherapy. Viral shedding may suggest intrapatient replication yielding a benefit and should be studied carefully in future studies.