A 4 10/12 year old girl with mental defect and multiple minor congenital anomalies was found to have a deletion of the short arm of chromosome 18. Limited investigation for loss of an allele did not reveal such a loss. Comparison of the phenotype of this patient with that of other patients with apparently identical cytogenetic abnormalities suggests a wide spectrum of variability in phenotype. Evidence suggests that the abnormal chromosome might not be a true telocentric chromosome.