The wide spectrum of anomalies associated with hemifacial microsomia (HFM) has made systematic and inclusive classification difficult. We propose a nosologic system in which each letter of the acronym O.M.E.N.S. indicates one of the five major manifestations of HFM. O for orbital distortion; M for mandibular hypoplasia; E for ear anomaly; N for nerve involvement; and S for soft tissue deficiency. The O.M.E.N.S. system is easily adapted for data storage, retrieval, and statistical analysis. A retrospective study of 154 patients with HFM classified according to the O.M.E.N.S. system confirmed the concept that the mandibular deformity is the cornerstone of the anomaly. Statistical analysis demonstrated a positive association between mandibular hypoplasia and the severity of orbital, auricular, neural, and soft tissue involvement. This study did not confirm a previously reported predominance of gender or sidedness. Analysis of statistical correlations failed to substantiate a Goldenhar variant as a syndromic entity. Our analysis showed that palatal deviation is probably caused by muscular hypoplasia and not by weakness of a particular cranial nerve.