Monozygotic twin brothers are described who share clinical features which include: moderate mental retardation, short stature, macrocephaly, frontal bossing, ptosis, low-set ears, brachydactyly, 5th fingers clinodactyly, single palmar creases, cryptorchidism, and prelingual sensorineural deafness. One of the twins presented with mild cardiac dilatation and died at age 3(1/2) from cardiac arrest during an episode of acute respiratory infection. While chromosome analyses performed for both twins on peripheral blood showed apparently normal karyotypes, screening for all telomeric regions on the surviving propositus revealed a combination of partial 6p trisomy and partial 11q monosomy. A balanced reciprocal translocation was found in the father. The phenotype of the twins is most likely related to this cryptic chromosomal rearrangement. The fact that the phenotype in this family partially overlaps with some previously reported phenotypes is discussed.