A 66-year-old woman heterozygous for a mutation in the ornithine transcarbamylase gene (Otc) participated in a phase I gene therapy trial for OTC deficiency. She received an adenovirus (Ad) vector expressing the functional OTC gene by intraportal perfusion. Fourteen years later she developed and subsequently died of hepatocellular carcinoma. A second subject, a 45-year-old woman, enrolled in the same trial presented with colon cancer 15 years later. We sought to investigate a possible association between the development of a tumor and prior adenoviral gene transfer in these two subjects. We developed and validated a sensitive nested polymerase chain reaction assay for recovering recombinant Ad sequences from host tissues. Using this method, we could not detect any Ad vector DNA in either tumor or normal tissue from the two patients. Our results are informative in ruling out the possibility that the adenoviral vector might have contributed to the development of cancer in those two subjects.