The diagnosis of unilateral condylar hyperplasia (UCH) requires a combined assessment consisting of clinical, radiological, and histopathological examination. The etiology of this condition is unknown. The purpose of this study is to report a rare case of UCH in a monozygotic twin. A 15-year-old girl was referred to our department complaining of facial asymmetry and malocclusion. Computed tomography and single-photon emission computed tomography imaging reveal enlargement of the left condyle and condylar neck, and an increased uptake that was diagnosed as active UCH. During the investigation of family and co-twin facial profile, no cases of UCH were identified. From the case reported in this study, the authors raise a hypothesis that can exist some environmental factor that is related in the development of condylar hyperplasia given the occurrence of this disease in one of twins.