Tetrasomy 18p is a rare chromosomal disease (1:140,000 live births), which affects females and males equally, and might be hereditary or caused by spontaneous changes (de novo formation) within the chromosome. The phenotype results from the presence of a small extra metacentric marker chromosome, an isochromosome 18p. The syndrome is characterized by mild-to-moderate mental retardation, poor language acquisition, seizures, microcephaly, short statue, minor facial dysmorphic features, congenital heart diseases, uro/renal malformations, abnormal muscle tone, spasticity of the lower limbs, and delayed ability to stand and walk. To our knowledge sensorineural hearing loss is described in the literature but has not been described as a typical phenotypic symptom of tetrasomy 18p.In the following report, a boy with tetrasomy 18p is described. In addition to psychomotor retardation with muscular hypotonia and orofacial dismorphysms, bilateral severe hearing loss was diagnosed. Thus, in all infants with known chromosomal aberration, early diagnostic procedures must be performed to unveil sensorineural hearing loss that might be overseen because of mental retardation. In particular, a brainstem-evoked response audiometry (BERA) should be considered for early diagnosis and treatment of possible hearing loss. Furthermore, in all children with developmental delay and dysmorphic features a chromosomal analysis should be initiated.