Gene therapy is being considered for the delivery of therapeutic proteins. We evaluated the delivery of the hormone erythropoietin (EPO) into cynomolgus macaques through intramuscularly administered adeno-associated virus (AAV) vectors. As expected, the animals developed supraphysiologic levels of EPO and polycythemia. However, severe anemia ensued in some animals because of an autoimmune response to endogenous and transgene derived EPO. This is the first example of gene therapy leading to inadvertent auto-immunity in primates.