An abnormal chromosome 21 is reported in a child with a phenotype strongly reminiscent of trisomy 21 syndrome. It results from duplication of the segment 21q21.fwdarw.21q22.2. Comparison of the phenotype with that of other partial and total trisomics shows that the characteristic features of the trisomy 21 syndrome (mongolism), the mental retardation in particular, is due to trisomy 21q22.1 and perhaps 21q22.2.