Since 1 January 1970, multiple births have been recorded by the Budapest Twin Register. The maternity wards report multiple births by telephone, retain the placenta, mark the umbilical cord of the first born and fill in the Registration Form of Multiple Births indicating the most important personal and obstetrical data. Placentae are examined by a pathologist. In like-sexed twins with dichorial placenta zygosity is determined 6-24 months after birth by applying blood and serum protein group determinations and dermatoglyphics. A total of 85.9 and 84.2% of all multiple births (356 and 316) were reported in 1970 and 1971, respectively. So far, the zygosity of twins was determined in 53.8 and 50.5% of all twins in 1970 and 1971.