In the present report we describe a severely mentally retarded and dysmorphic female child with a de novo 3q/7q reciprocal translocation and loss of band 7q35. This finding supports the hypothesis that the occurrence of mental retardation and/or congenital malformations in de novo autosomal reciprocal translocation may be due to the loss of a small amount of chromatin material during this chromosomal rearrangement.