This is a report of an additional patient affected by hemifacial myohyperplasia (HMH). We postulate that this condition originates around the fourth gestational week at any step of cranial muscle development from somitomeres to branchial arches, most probably due to prolonged period of proliferation during cranial muscle development, subsequent abnormal contact between cranial neural crest (CNC) cells and cranial myoblasts, and an impaired interaction among CNC cells and cranial myoblasts derivatives. HMH may represent another example of somatic mosaicism and its features can be explained by a combination of morphostatic and morphodynamic mechanisms of pattern formation during development. Here we suggest that HMH is a sequence in which the primary defect is hyperplasia of the facial muscles and the other findings are secondary to this. © 2010 Wiley-Liss, Inc.