We report on a 15-year-old black boy with severe mental retardation, multiple congenital anomalies, and a supernumerary ring chromosome mosaicism. Fluorescence in situ hybridization with a chromosome 1 painting probe (pBS1) identified the ring as derived from chromosome 1. The karyotype was 46,XY/47,XY,+r(1)(p13q23). A review showed 8 reports of ring chromosome 1. In 5 cases, the patients had a non-supernumerary ring chromosome 1 resulting in partial monosomies of the short and/or long arm of chromosome 1. In 3 cases, the presence of a supernumerary ring resulted in partial trisomy of different segments of chromosome 1. In one of these cases the supernumerary ring was composed primarily of the centromere and the heterochromatic region of chromosome 1, resulting in normal phenotype. Our patient represents the third report of a supernumerary ring chromosome 1 resulting in abnormal phenotype.