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Microgeographic variations in the frequency of consanguineous marriages and historical change in inbreeding were analysed in an Italian mountain population. From 1565 to 1980 in the 35 parishes of the Upper Bologna Appennine 3532 dispensations for marriages up to the third cousin degree of relationship were recorded in a total of 43252 marriages. The average frequency of consanguineous marriages was 8.17% and the mean inbreeding coefficient was a — .00125. Within the mountain region marked microgeographic variations in inbreeding related to environmental and geographic factors were observed. There are microgeographic differences in pedigree structure. As a consequence of female virilocal migration the tendency for a higher frequency of pedigrees with more intermediate male ancestors was reduced in the more inbred and more isolated upper valley sub­population. In the second half of the nineteenth century historical change in in- breeding values was characterized by a sudden increase in close consanguineous marriages. During the last decades of this century the decrease in consanguineous marriages was connected with the phenomenon of the breakdown of isolates. A high correlation (r = .98) between inbreeding increase and population growth has been stressed suggesting that the secular trend in inbreeding may depend on demographic factors in addition to social and historical events.