Both replication-incompetent and replication-selective adenoviruses are being developed for the treatment of cancer and other diseases. Concerns have been raised about the safety of intra-vascular adenovirus administration following a patient death on a clinical trial with a replication-defective adenovirus. In addition, the feasibility of vascular delivery to distant tumors has been questioned. dl1520 (ONYX-015) is a replication-selective adenovirus that has previously shown safety and antitumoral activity following intratumoral injection. This is the first report of intra-vascular administration with a genetically engineered, replication-selective virus. A phase I dose-escalation trial was performed in patients with liver-predominant gastrointestinal carcinoma (n = 11 total; primarily colorectal). dl1520 was infused into the hepatic artery at doses of 2 x 10(8)-2 x 10(1)2 particles for two cycles (days 1 and 8). Subsequent cycles of dl1520 were administered in combination with intravenous 5-fluorouracil (5-FU) and leucovorin. No dose-limiting toxicity, maximally tolerated dose or treatment-emergent clinical hepatotoxicity were identified following dl1520 infusion. Mild to moderate fever, rigors and fatigue were the most common adverse events. Antibody titers increased significantly in all patients. Viral replication was detectable in patients receiving the highest two doses. An objective response was demonstrated in combination with chemotherapy in a patient who was refractory to both 5-FU and dl1520 as single agents. Therefore, hepatic artery infusion of the attenuated adenovirus dl1520 was well-tolerated at doses resulting in infection, replication and chemotherapy-associated antitumoral activity.