Objective: To determine the proportion of hemifacial microsorma (HFM) in patients with unilateral or bilateral "isolated" microtia.Design: Prospective cohort clinical study.Setting: University-affiliated, tertiary referral clinic for patients with microtia.Patients: One hundred consecutive patients with isolated microtia.Interventions: All the patients underwent a clinical examination and audiologic evaluation. The OMENS classification system was used to grade the severity of craniofacial features: orbital deformity, mandibular hypoplasia, ear deformity, nerve (cranial nerve VII) involvement, and soft-tissue deficiency. Each anatomical abnormality was graded from 0 (normal) to 3 (most severe) (score range, 0-15).Main outcome Measures: The OMENS scores, percentage of patients with isolated microtia and undiagnosed HFM, and isolated microtia as an early clinical marker for HFM.Results: Forty patients (40%) with microtia were determined to have HFM (31 unilateral and 9 bilateral). Mean patient age was 9.2 years (range, 6 weeks to 41 years), with male predominance (27 males and 13 females). The OMENS scores were less than 5 in 24 patients and 6 to 10 in 16 patients. Thirty patients had cranial nerve deficits, and 37 had mandibular asymmetry. Thirty-seven patients demonstrated conductive hearing loss, and I had sensorineural hearing loss.Conclusions: Isolated microtia served as an early clinical marker for asymmetrical facial growth in 40% of the patients. Isolated microtia and HFM could represent a spectrum of expression of the same developmental phenomenon.