Recently, [123I]Ro16-0154 has been introduced as a potential benzodiazepine (BZD) receptor imaging tracer for SPECT. We investigated the biodistribution of [123I]Ro16-0154 in humans and compared it with the animal data. There was a rapid and high 123I-uptake in the brain comparable to rCBF. In the following images, however, regionally different changes were observed, resulting in a subsequent intracerebral retention parallel to the known distribution of BZD receptors. The results suggest that [123I]Ro16-0154 is of potential clinical use as a BZD receptor imaging agent.