A 13q isodisomy in a balanced karyotype: 45,XY,-13,-13, + i(13)(q10) was found in cultured amniocytes studied because of advanced maternal age. The isochromosome was monocentric and a new mutation as both parents had normal chromosomes. Fetal blood was studied to exclude 13-trisomy mosaicism. All (100) lymphocytes studied had the same karyotype with i(13)(q10) as the amniocytes. To determine the origin of the isochromosome, six microsatellite markers from 13q were analysed: D13S175, D13S166, D13S162, AC224, COLAC1 and D13S122. The results indicated that the i(13)(q10) was of paternal origin and isodisomic. The father had a risk of 1/20 for being a carrier for an autosomal recessive, progressive brain disorder, variant late infantile neuronal ceroid lipofuscinosis (CLN5). The risk for the fetus for this disorder of chromosome 13 was excluded by haplotype analysis. A healthy child was born at week 40 of pregnancy, supporting the idea that there are no paternally imprinted genes on chromosome 13q. Analysis of extra embryonal tissue (four samples studied) revealed the same balanced karyotype with the i(13)(q10)pat chromosome. According to the cytogenetic and molecular studies, the origin of the isochromosome 13 could be a transverse centromere cleavage at the paternal meiosis II or at an early mitosis.