The immunological nature of allergic diseases is a recognized fact, but their pathogenesis is still largely unknown, representing one of the less understood areas of immune pathology.

Twin studies in allergic diseases have been carried out before (Gedda and Teodori, 1962), contributing to prove the great importance of heredity in the occurrence of allergic disease.

Yet, the nature and mode of inheritance of allergic hereditary factors is still unknown, thus representing further question marks in the mystery of allergic pathogenesis.

The present study has been extended to include allergic diseases within the wider scope of total pathological experience, through a comparison between a sample of twin pairs with a history of allergic disease and a control sample of nonallergic twins.

All our data have been derived from the records of the Twin Register of the Mendel Institute. These records have been coded and transferred onto IBM punched cards with the support of the Italian National Research Council (Gedda and Milani-Comparetti, 1966). A population of 500 twin pairs was selected for having been included in the Twin Register before 1955, so as to have a minimum age of 15 years. The case histories of all these pairs had been coded and punched independently, according to a coding system developed in our Institute, which listed the following items: (1) oculorhinitis; (2) asthma; (3) urticaria; (4) allergic eczema; (5) unspecified allergic disease.

As many as 76 of our 500 pairs were found to have a positive history for at least one of these classes of disease, representing an incidence of 15.02% in our material.