Partial trisomy for the distal part of 15q due to a balanced maternal translocation t(X;15) is described in a 21-month old girl with growth and psychomotor retardation and a cranio-facial dysmorphism ressembling that of a previously reported patient. Treatment of lymphocytes with BrdU has shown inactivation of the normal X in the mother, and inactivation of either the abnormal or the normal X in the proposita. When the abnormal X was inactivated, the extent of inactivation of the autosome was variable.