Clinical pharmacology can have an important impact on the development of orphan drugs, i.e., therapeutic agents for rare diseases. This topic was explored at a US Food and Drug Administration (FDA) Advisory Committee meeting in March 2011.1 At that meeting, preliminary data were presented regarding the breakdown of the clinical pharmacology content in the approvals for orphan drugs in 2006–2010.These data, along with follow-up observations, are presented here, focusing on the in vivo study content and the relevant populations.