An essential element in any investigation of environment-genetics interaction is twin registries. At present there exist two types, one based on the whole population and another relying on voluntary participation. The second type is an important source of epidemiological information regarding the presence or absence of a disease (concordance) between members of monozygotic and dizygotic pairs. However, this type presents two serious drawbacks: the data do not cover all twins, and since the twins enrolled are those who require the greatest assistance, this results in a serious selection bias [2]. On the contrary, the first type has neither of these defects and represents an invaluable source of information for epidemiological studies on the concordance of pysiological features, morbidity and mortality, together with genetic and environmental influences and their interaction [1, 3-5].

April 1993 saw the first enrolment in a Multiple Pregnancies Registry designed to collect data from 47 Italian hospitals. This registry forms part of a larger survey, the “Mercurio Project”, originally set up in April 1992 under the aegis of the Italian Multicentre Birth Defects Registry (IPIMC). The aims of the registry are the following:

(a) to set up longitudinal epidemiological studies on the health, morbidity and mortality of twins

(b) to assess the role of environmental-genetic interaction in determining childhood behaviour and disease (using case-control studies where each twin is matched with his or her cotwin)

(c) to draw the attention of socio-sanitary personnel to twins and their special needs.