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OBJECTIVE--To study the genetic contribution to the aetiology of insulin dependent diabetes mellitus. DESIGN--Historical cohort study of twins, with information on diabetes being gathered by questionnaire, verification of the diagnosis by the subject's diabetologist or general practitioner, and clinical examination in available twins. SETTING--Danish twin register and diabetic clinics and general practices throughout Denmark. SUBJECTS--20,888 twin pairs born during 1953-82, included in a population based nationwide register. MAIN OUTCOME MEASURES--Crude and cumulative concordance rates and heritability in monozygotic and dizygotic twins. RESULTS--The crude probandwise concordance rate was 0.53 (95% confidence interval 0.33 to 0.73) for monozygotic twin pairs and 0.11 (0.05 to 0.21) for dizygotic twin pairs. When adjusted for age at onset of diabetes and age at last observation among unaffected twin partners the cumulative proband-wise risk from birth to age 35 was estimated as 0.70 (0.45 to 0.95) for monozygotic twins and 0.13 (0.05 to 0.20) for dizygotic twins. The correlations of liability for monozygotic and dizygotic twin pairs were estimated as 0.96 (SE 0.09) and 0.58 (0.07), with a heritability estimate of 0.72 (0.21). CONCLUSIONS--The risk of insulin dependent diabetes in monozygotic twins is higher than previously thought and for dizygotic twins is higher than in ordinary first degree relatives. Based on the findings of this study the genetic component to the disease seems more important than hitherto believed.