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A family is described in which a mother and two of her children were mosaic for a small supernumerary ring chromosome. As the origin of the ring chromosome could not be determined by routine cytogenetic studies, fluorescent in situ hybridization was performed, which indicated that the ring chromosome was derived from the pericentromeric region of chromosome 7. Further characterization with a YAC-probe showed the involvement of the proximal q-arm of chromosome 7. Both sibs had speech difficulties and were mildly mentally retarded whereas the mother's intelligence was at the lower end of the normal range. They all had an unusual face, characterized by a flat profile, short forehead, downslant of the palpebral fissures, high and broad nasal bridge, simply formed ears, and prognathia. This is the second report of a small supernumerary ring chromosome derived from the pericentromeric region of chromosome 7, and the described clinical phenotype differs from that delineated in the previous report.