Three sibs with similar phenotypic abnormalities were found to have partial trisomy of chromosome 18 and a sizeable deficiency of one of the G chromosomes. The mother and her father as well as five other maternal relatives are carriers of a reciprocal translocation. A total of 35 individuals were tested. Viability in spite of a large deficiency may be explained by the distribution of heterochromatin and euchromatin in the acrocentric chromosome. The recovery of only one combination from adjacent segregation is probably the result of parallel orientation of a chain of four and/or early zygotic lethality. Blood grouping studies showed that the loci for Rh, MN, Duffy and Kidd are not situated on the deleted portion of the G chromosome involved.