The object of this work is to try to elucidate the role of genetic and environmental factors in the aetiology of diabetes by studying a series of identical twins. Concor&ince in identical (monozygotic) twins does not necessarily mean that a disease is genetic in origin. Twins usually live together in early life and thus share the same environment. Concordance could therefore be the result of genetic or environmental similarity. However, in later life most twins live apart and then concordance does suggest a genetic disease. Discordance, on the other hand, must indicate that a disease is due, at least in part, to nongenetic factors. We are therefore particularly interested in discordance in younger and concordance in older twins.