We report on a boy with two supernumerary marker chromosomes which were identified by fluorescencein situhybridization and derived from chromosome 6 and 9. In lymphocytes, a mosaic karyotype was found: 46,XY (17%)/47,XY,r(6) (24%)/47,XY,r(9) (20%)/48,XY,r(6),r(9) (39%). Only minor dysmorphic features and mild developmental delay were present. Despite extensive fluorescencein situhybridization studies using a large panel of probes, we were unable to characterize the marker chromosomes in more detail, mainly because no probes for the chromosome regions involved were available to us. In order to reach a better understanding of the clinical relevance of small supernumerary marker chromosomes, it will be necessary to create a widely available set of probes, covering all chromosome regions.