Two similar pericentric inversions of chromosome 9 are transmitted in two unrelated families, each of which includes a consanguineous marriage. In both instances the inverted segment comprises the centromere region and the proximal part of the long arm, including the secondary constriction. The inverted chromosome was found in one homozygote and in three heterozygotes of the six karyotyped individuals of one family; in the second family, three of five karyotyped individuals were heterozygotes. The health problems of the propositi are apparently not associated with their chromosomal constitution.