Chromosome anomalies are responsible for a significant proportion of patients with mental retardation, and congenital anomalies. Development of new molecular cytogenetic techniques has provided a powerful tool for detection of patients with subtle chromosome abnormalities. Particularly, investigation of the gene-rich subtelomeric regions has generated interest regarding the implications and prevalence of cryptic chromosomal rearrangements. Here we describe an adult with a submicroscopic deletion of 18pter, detected by subtelomeric FISH probe. The patient is a 42-year-old man with a history of developmental delay, moderate mental retardation, and symptoms of paranoid schizophrenia since adolescence. His physical examination is remarkable for only a few dysmorphic findings typically seen in 18p− syndrome (round face, hypertelorism, down-slanted palpebral fissures, temporal narrowing, small hands and feet). He lacks significant short stature, skin changes, and associated anomalies involving internal organs. All known patients with deletions of the short arm of chromosome 18 have either loss of large parts of 18p or of the entire p-arm, or have complex chromosomal rearrangement involving other chromosomes. To our knowledge, this is the first description of a cryptic subtelomeric deletion of 18p and the first case of such a chromosomal anomaly in a patient with schizophrenia. Small subtelomeric chromosomal deletions would be missed by standard G-banded karyotyping. Therefore, FISH analysis using subtelomeric probes should be considered for diagnostic evaluation of patients with psychiatric symptoms and mental retardation in whom the karyotype is normal. © 2003 Wiley-Liss, Inc.