First-generation GD2-CAR T cells can be detected in the peripheral blood of patients for a prolonged period of time. Real-time quantitative PCR was used to assess the presence and persistence of CAR–T cells. GD2-ATCs or GD2-CTLs could be detected, at or after 6 weeks after infusion, in the circulation of 11 of 19 (58%) patients. Although the levels of detection were low, transgenic signals could be identified at 96 weeks for CAR-CTLs and 192 weeks for CAR-ATCs after infusion. The estimate of frequency of CAR– T cells was obtained using standard curves of DNA from tumor cell lines transduced with the retroviral vectors encoding each CAR. Integrant analysis showed between 6 and 8 proviral integration sites per cell. Sensitivity assays in which transduced T cells were diluted with nontransduced T cells showed unequivocal detection ability when 1 transduced cell was diluted in 1000 to 6000 nontransduced cells, corresponding to 0.0001% to 0.002% of the tumor cell lines used for the standard curve.