Clinical, cytogenetic and other laboratory studies are reported in a set of dizygous twins, their parents and siblings. The proposita was a girl with profound mental deficiency of unknown cause; she had been born with a placenta showing marked chorangiomatosis. In addition, she was found to have a chromosomal error consisting of a translocation of material of the long arms of a No. 2 chromosome to the long arm of a No. 15 chromosome. Studies of normal marker loci failed to provide evidence that any of them had been lost or inactivated as a consequence of the translocation.