Studies were made of the somatic chromosome constitutions of a group of 14 mongoloids, including two who were sibs, together with the available members of their immediate families. In eight cases the classical trisomy for chromosome 21 was the only abnormality observed. The mongoloid sibs fell into this category. In the remaining six cases, however, some other abnormality was also observed in the mongoloid family. The abnormalities seen include an unusually long Y chromosome, an unusually long small acrocentric, an abnormal chromosome 18, and translocations or deletions in some portion of the karyotypes. The high frequency of abnormal chromosomes in these families suggests that these “minor” abnormalities may increase the frequency of nondisjunction of other chromosomes during meiosis, thus increasing the probability of mongoloid offspring in these families.