Abstract The development of digit span was studied in relation to mental age in two large groups of severely subnormal (SSN) individuals, one diagnosed as suffering from Down's syndrome and the other of mixed aetiology, and a group of normal subjects matched for mental age. Digit span in the Down's syndrome and SSN mixed aetiology groups was generally lower than that expected on the basis of mental age. A cross-sectional and longitudinal study showed a failure of development of digit span in both SSN groups which resulted in an increasing lag between digit span and mental age as mental age increased. Some possible explanations for the lack of short-term memory development in Down's syndrome and other SSN subjects are discussed, together with the possible implications of this for other aspects of cognitive development.