A chromosome 13 with extra material on the short arm was observed in a 17-year-old boy showing defects in skeletal growth, an altered hormone profile and asthenoteratozoospermia, and in a 46 XX fetus subjected to prenatal diagnosis. The abnormal chromosome 13 had been transmitted from phenotypically normal parents who were the mother (case 1) and the father (case 2). The extra material on the abnormal chromosome 13 was brightly fluorescent after Q-banding, and positive in C-banding (CBG) and distamycin A-Dapi (DA-Dapi) banding. Staining of the nucleolus organizer region indicated its retention. In-situ hybridization of a Yq-specific repetitive DNA probe to chromosomal spreads from both cases demonstrated that the der(13) chromosome contains sequences of the Yq heterochromatic region. However, the apparently identical unbalanced (Y;13) translocation may either interfere (case 1) or not (father of case 2) with meiotic or postmeiotic sperm cell development.