The relative roles of genetic and environmental factors in sciatica were studied in the nationwide Finnish twin panel consisting of 9365 adult pairs of the same gender. Morbidity was analysed from two sources of data: the life-long cumulative incidence was measured by a postal questionnaire, and the rate of hospital admission during a 14-year period was measured by record-linkage of the twin panel and the nationwide hospital registry. Altogether 2220 individuals reported sciatica diagnosed by a doctor and 304 were admitted to hospital with a diagnosis of sciatica. The proportion of concordant pairs (calculated from affected pairs) was 17.7% for monozygotic and 12.0% for dizygotic pairs in the life-long cumulative incidence of reported sciatica, and correspondingly 4.6% and 1.9% for those admitted to hospital (a 14-year period) because of sciatica. The estimated heritability was 20.8% for those with reported sciatica and 10.6% for those admitted to hospital. The results show that environmental factors account for more than 80% of the etiology of sciatica, and more than 90% in the case of patients admitted to the hospital. Genetic factors, however, were relatively more significant in individuals under 40.