We report on a 3-year-old male with developmental delay, autistic behavior, and minor abnormalities consistent with trisomy 8 syndrome whose cytogenetic analysis revealed mosaicism for a supernumerary ring chromosome (SRC). Fluorescence in situ hybridization (FISH) studies, using centromeric and yeast artificial chromosome (YAC) probes, were performed to characterize further the supernumerary chromosome. The ring origin has been detected from the short arm of chromosome 8, resulting in r(8)(p10p23.1). Moreover, uniparental disomy (UPD) using microsatellite analysis was excluded. To our knowledge a total of 25 cases, confirmed by FISH, have been reported with either supernumerary marker or ring chromosome 8. We present a detailed clinical and molecular cytogenetic characterization of this additional case in order to better define the genotype-phenotype correlation.