In this study, a four-generation Chinese family in Nanyang, Henan Province was identified with partial trisomy 9p syndrome. Of the 23 family members studied, six were characterized with mental retardation and mild facial and little finger anomalies. All affected family members demonstrated significant intrafamilial homogeneous phenotype except concomitance epilepsy in proband. On the basis of G-banding, the proband showed a translocation between chromosomes of 9p and 21q and partial 9p trisomy. The karyotype was 46, XY, der (21) t (9; 21) (9p22.2; 21q22.3) pat. Further karyotyping of other affected members and their patients in this family revealed translocation of chromosomes of 9p and 21q, with partial 9p trisomy in all affected members. The partial 9p trisomy was the direct result of abnormal segregation of a balanced translocation cell between chromosome 9 and 21 in one of the parents. The extra distal half of the short arm of chromosome, 9pter-->9p21, is responsible for the major clinical features such as mental retardation and mild facial anomaly. The cause of epilepsy in proband was discussed.