An autosomal reciprocal translocation (12;21) was found in five seemingly unrelated families in Finland. Three families had had multiple spontaneous abortions and two families had a child with Down's syndrome. The genealogies of the five families were traced using population registries, and four families were found to have a common ancestor born in 1752. Kinship to the fifth family could not be established but its ancestors were traced back to the same rural parishes as those of the four other families. The translocation segregated at the same frequency as normal chromosomes. A statistically insignificant increase in spontaneous abortions was detected when the matings of translocation carriers were compared with non-carrier matings. The increase may however be clinically significant. These results permit more accurate counselling in these and similar translocation families.