The etiology of simple goiter, affecting up to 5% of a population in nonendemic areas, is incompletely understood. It is generally believed to be multifactorial in origin, but the relative contributions of genetic and environmental factors remain to be clarified. Therefore, we investigated a well defined population of Danish twins. We performed a historical cohort study of 5.479 same sex twin pairs born between 1953 and 1972. Information on goiter was obtained from a nationwide questionnaire survey in 1994. Information from hospitals, out-patient clinics, and the subjects' general practitioners was sought to verify the diagnoses. Concordance rates, tetrachoric correlations, and heritability were determined. The crude probandwise concordance rates were 0.42 [95% confidence interval (CI), 0.26-0.591 and 0.13 (95% CI, 0.06-0.24) for female monozygotic and female dizygotic pairs, respectively. The age-adjusted cumulative probandwise risk for simple goiter from birth to age 43 yr was 0.53 (95% CI, 0.23-0.83) for female monozygotic twins and 0.18 (95% CI, 0.05-0.35) for female dizygotic twins (P = 0.003). The tetrachoric correlations were substantially higher in monozygotic (0.82; SE, 0.07) than in dizygotic twins (0.47; SE, 0.12). Model-fitting analysis suggested that the heritability of the liability to the development of simple goiter in women is approximately 82%. Individual-specific environmental factors not shared by cotwins seemed to explain the remaining 18%. We conclude that the etiology of clinically overt simple goiter is multifactorial. Genetic factors play a major role in the etiology of simple goiter in females, but environmental factors are also of importance.