A malformed infant and her phenotypically normal relatives were studied cytogenetically. Mosaicism consisting of normal cells and cells carrying an abnormally long chromosome 16 was detected in members of three generations of the same family. The abnormal chromosome presented a terminal secondary constriction and an extra chromosome segment. Breakage of the abnormal secondary constriction and separation of the extra chromosome segment from the abnormal No. 16 occurred in all the carriers in the family. The proband also presented selective endoreduplication of the abnormal segment with subsequent successive separation of the extra fragments formed. The cytogenetic mechanisms involved in the production of these chromosome aberrations and their possible phenotypical consequences are discussed.