A constitutional karyotypic abnormality, ins(16)(p11.2), is described in a case of myelodysplastic syndrome (MDS). The source of material for this insertion could not be established, but did not arise from either a balanced deletion or translocation, and did not consist of constitutive heterochromatin as defined by C-banding. The same lesion was found in both sisters, both nephews and four of the five great-nephews. Of these, all were phenotypically and haematologically normal, with the exception of a great-nephew who at the age at the age of 6 exhibits features compatible with partial trisomy 16p. The relationship of the karyotypic abnormality to the MDS and partial trisomy 16p in this family is discussed.