An E1/E2a/E3-deficient adenoviral vector encoding an epitope-tagged (flagged) human factor VIII (FVIII) cDNA was delivered systemically to four cynomolgus monkeys. Analysis of liver biopsy samples revealed the presence of vector DNA at all points in the study (day 7, 28, and 56), with vector copy number declining approximately 10-fold between day 7 and day 56. Immunoprecipitation/Western analyses detected human flagged FVIII in the plasma of all monkeys and expression persisted for 14-28 days. Peak plasma FVIII levels ranged from 50 to 100 ng/ml. Bethesda assays revealed no inhibitor in two animals, the development of a low-level transient inhibitor in one animal, and an inhibitor titer that continued to increase for the duration of the study in one animal. Other treatment-related changes included modest increases in liver enzymes, an increase in interleukin-6 (IL-6) levels, and a transient decrease in platelets in all four animals. These data indicate that early generation adenoviral vectors do not support the long-term expression of FVIII in nonhuman primates.