A clinically normal mother of three retarded children has been determined by G-banding to have a balanced translocation 46,XX,t(13;20) (q34;p11.2). The children each have an unbalanced form of the translocation with partial trisomy for 20p. Extensive gene marker studies have been unable to affix any specific gene locus onto the short arm of chromosome 20. The balanced translocation was inherited from the maternal grandfather. Two phenotypically abnormal deceased members of the family are believed to have had the unbalanced trisomy 20p condition. An increases number of spontaneous abortions were possibly due to lethal unbalanced 20p deletions. The moderate to mild mental retardation and somewhate unusual features (round face, prominent cheeks and nose, short mandible) in the three siblings and two other affected relatives suggest that trisomy of the short arm of chromosome 20 may cause a distinguishable clinical syndrome. Vertebral abnormalities and abnormal dermatoglyphics are part of the picture. Clinical and cytogenetic findings of all reported cases are compared.