The purpose of this review is to bring out the clinical value of a twin pair, when a disease strikes one or both partners. A “Clinical Cotwin Method” is thus described.

In the case of concordant disease in MZ twins, the points of interest suggested by this method are the study of (1) the etiopathogenesis of the disease; (2) the genetic variability in the manifestation of such disease; (3) the variability of expression of the morbid genotype; and (4) the chronogenetic variability, that is, the possible difference between times of onset of disease, of its symptoms, and of its outcome. In the case of discordant disease in MZ twins, either one twin is ill and the other healthy, or else, twins have two different diseases. In the first case, the clinician finds an excellent basis of comparison in the healthy twin, which allows him to extend his knowledge of the morbid phenotype and to discover minimal symptoms which would otherwise go unobserved. In the case of two different diseases which strike MZ twins simultaneously, comparative analysis of the general symptoms is possible, which allows a deeper knowledge of the etiopathogenesis of the morbid processes. DZ twins also are of high scientific interest because their semiequivalent genotypes have simultaneous times of departure. This situation differs from that of full sibs and that of half sibs; it is also very useful to analyze the disease that DZ twins face in the same physical time. Both twin care and scientific research can be greatly helped by hospital department of twins, as well as outpatient departments or consultants for occasional and routine visits.