We identified 10 members of a single family with mental retardation and microcephaly, one member having macrocephaly instead. The pedigree was best compatible with the segregation of a small translocation, despite results of previous cytogenetic studies in several relatives being apparently normal. Eventually high resolution and fluorescence in situ hybridization studies in the parents allowed the detection of a balanced translocation between 5qter and 6qter, and of its unbalanced products in the offspring. The pertinent findings from the family are briefly compared with the clinical findings in patients from the literature with either a duplication or deletion of 5q35-ter, or a duplication or deletion of 6q27-ter.