The term hemifacial microsomia refers to unilateral defects in development of structures derived from the first and second branchial arches. Recently we evaluated three unrelated children who had a similar pattern of unilateral craniofacial defects that was associated with other structural abnormalities having a disruptive vascular pathogenesis. The clinical findings in these patients suggest that one cause of hemifacial microsomia is in utero interruption of blood flow.