UNLABELLED: A first-in-human phase 1 clinical study was performed on 12 healthy adults with a high-specific-activity carrier-free formulation of (123)I-iobenguane. Clinical data are presented on the behavior of this receptor-targeting imaging agent. METHODS: Whole-body and thoracic planar and SPECT imaging were performed over 48 h for calculation of tissue radiation dosimetry and for evaluation of clinical safety and efficacy. RESULTS: A reference clinical imaging database acquired over time for healthy men and women injected with high-specific-activity (123)I-iobenguane showed organ distribution and whole-body retention similar to those of conventional (123)I-iobenguane. The heart-to-mediastinum ratios for the high-specific-activity formulation were statistically higher than for conventional formulations, and the predicted radiation dosimetry estimations for some organs varied significantly from those based on animal distributions. CONCLUSION: Human normal-organ kinetics, radiation dosimetry, clinical safety, and imaging efficacy provide compelling evidence for the use of high-specific-activity (123)I-iobenguane.