It is calculated that distal deletion of the short arm of chromosome 1 occurs in one out of every 5000 live births and causes approximately 1.2% of cases of mental retardation of unknown origin. This alteration usually cannot be detected in the standard karyotype, requiring molecular cytogenetic techniques for the diagnosis. In addition to the neurological manifestations, it may cause internal organs malformations, such as congenital heart disease, and a characteristic facial phenotype. This report describes the clinical and cytogenetic findings from the first two cases diagnosed in Mexico, confirmed by fluorescence in situ hybridization test, and compares them to those described in the literature. The probable subdiagnosis of this entity, the importance of improves its recognition and the useful data for the clinical suspicion are also discussed.