Six probands, apparently not related, with a minimal phenotype of Down's syndrome were investigated between 1970 and 1984 in our laboratory. We found in all of them an identical chromosomal abnormality 46,XX or XY,-21,+ der21(dupq22delp23). The der 21 was due to aneusomie de recombinaison, each mother having an abnormal chromosome 21: inv(21)(p12;q22). The fathers' caryotypes were normal. All parents were young and healthy. Pedigrees were established in order to find a relationship between these families. Four of our probands could be related. Familial investigations are still in progress for the last two cases; the ancestors being born in the same small geographical area (within 50 km2) we think that we shall be able to establish a relationship with the others families.