Idiopathic mental retardation is a common condition the origins of which are poorly understood. Following initial reports that small chromosomal rearrangements affecting telomeres could be an important aetiological contributor, several new methods for screening patients have been developed. Over the past few years, 22 studies have reported results from 2585 patients. The prevalence of abnormalities in the entire group is 5.1%; but the figure is higher (6.8%) in individuals with moderate to severe mental retardation. About half the cases are caused by a de novo deletion, and about half by a balanced translocation segregating in a patient's family. Despite the large sample size available, it is still not clear whether a combination of thorough clinical examination and assiduous cytogenetic investigation might not be as effective at detecting subtelomeric anomalies as molecular assays.