Evidence is presented of a higher than normal incidence both of clinical coeliac disease and of small-intestinal mucosal abnormalities in relatives of children with coeliac disease. In such relatives the incidence of mucosal abnormality may differ from the incidence of clinical coeliac disease. The data show an absence of any simple Mendelian pattern of inheritance: in place of the hypothesis that inheritance is through a dominant gene of reduced penetrance, it is argued that the pathogenesis of coeliac disease is multifactorial, the genetic basis of susceptibility being polygenic and interacting with environmental factors. On this hypothesis the relative contributions of inheritance and environment to liability to the clinical condition are estimated, the genetic component being 45% ± 9. Environmental factors appear more important in the development of mucosal abnormality.