We report the death of an 18-year-old male with partial ornithine transcarbamylase (OTC) deficiency who participated in a pilot (safety) study of gene therapy. The vector used for this trial was based on human adenovirus type 5, deleted in E1 and E4, and contained human OTC cDNA. It was infused into the right hepatic artery at a dose of 6x10(11)particles/kg. Approximately 18 h. following gene transfer the subject was noted to have altered mental status and jaundice--clinical signs not seen in any of the first 17 subjects in this study. Subsequently, his clinical course was marked by systemic inflammatory response syndrome, biochemically detectable disseminated intravascular coagulation, and multiple organ system failure, leading to death 98 h following gene transfer. Post-mortem examination was consistent with the clinical course, and vector DNA sequences were readily detectable in most tissues. The subject had high serum levels of IL-6 and IL-10 but normal TNFalpha immediately after infusion of the vector. This experience points to the limitations of animal studies in predicting human responses, the steep toxicity curve for replication defective adenovirus vectors, substantial subject-to-subject variation in host responses to systemically administered vectors, and the need for further study of the immune response to these vectors.