On the basis of the numbers of single and multiple births routinely collected by the Central Institute of Statistics, trends in multiple births in Italy over the period 1955-1983 were analyzed. Between 1955 and 1983 the frequency of multiple births declined by about 25% (from 12.6/1000 deliveries to 9.6/1000 deliveries). The downward trend was constant until the early 1970s when rates tended to level off and increase slightly. This finding was largely attributable to trends in dizygotic rates, monozygotic births being approximately constant over the period considered. Multiple birth rates rose till age 35-39, being more than two times higher in this age group than in teenagers, but flattened off in the subsequent strata of age: this finding was constant over the period considered. Despite the general decreasing trend, the regional differences persisted largely unchanged, multiple birth rates being about 30% higher in Southern (and less developed areas) of the country than in the North of Italy. Geographic differences were limited to dizygotic pregnancies, monozygotic rates being constant (about 4/1000 pregnancies) in various areas.