Dermatitis herpetiformis (DH) and coeliac disease (CD) are gluten-sensitive diseases which have a common immunogenetic background, with the histocompatibility locus antigen (HLA) alleles DQ A1*0501 and B1*0201 in the short arm of chromosome 6. CD is well known to cluster in families whereas DH has not been generally regarded as a familial disease. To study the familial incidence of DH, a prospective study was started in 1969 at the Department of Dermatology of Helsinki University Hospital and in 1976 at the Department of Dermatology of Tampere University Hospital. A total of 1018 patients with DH were diagnosed and questioned for positive family histories. Of the 999 unrelated DH patients, 105 (10.5%) had one or several affected first-degree relatives. The disease in the relatives was either DH (4.4%) or CD (6.1%). Analysis of the 105 families showed that 13.6% of parents, 18.7% of siblings and 14.0% of children were affected, a segregation pattern which fits well to a dominant mode of Mendelian inheritance. Gender may also be important because the first-degree relatives affected with DH were more often females and those affected with CD twice as often females as males. In contrast, there were more males among the 105 propositi with DH and also among the 894 DH patients with no affected relatives. The present study clearly shows that DH is a familial disease in which the first-degree relatives can be affected both with DH and CD, presumably because of a common genetic background. The environmental factors which could cause the rather high penetrance of DH and CD in the first-degree relatives of DH patients remain unknown.