Early loss of one fetus in a multiple gestation as a 'vanishing' twin is a well recognized phenomenon. It is uncertain whether this has an impact on the development of the surviving co-twin. The aim of this study is to compare the development of singletons, twins and the surviving co-twins of a vanishing twin. The 324 children born to 229 women who were recruited into the study between 1999 and 2001 formed the study population. Children were assessed at 1 year of age with Griffiths Mental and Developmental Scales. A neurological examination was performed using an optimality score to exclude those with severe neurodisability. Three hundred and five children (92 singletons, 180 twins and 33 survivors with a vanishing twin) were included. The sub- and general quotient scores in singletons and surviving co-twins of a vanishing twin did not differ significantly. Twins had significantly lower scores than singletons in all areas of development and were more likely to be born early with lower birthweights. Following adjustment for gestation and birthweight, the difference between the two groups was nullified suggesting that the slower development of twins is related to their prematurity and lower birthweight.