Cases of secondary tumours have been reported as a possible adverse reaction to the treatment of certain blood cancers with CAR T-cells. A recent analysis of the cases reported to the Paul-Ehrlich-Institut highlights the challenges in the assessment of these newly diagnosed T-cell diseases. The result: in order to better assess these rare but serious cases and identify possible risk factors, CAR T-cell-specific assessment criteria should be defined in pharmacovigilance activities and specific molecular test methods should be established. These steps will further increase patient safety.