Objective To explore the relationship between complicated clinical situation and genetic etiopathogenisis of a girl with growth retardation and mental retardation. Methods High resolution G-banding was used to examine karyotypes of the girl and her parents. Fluorescence in situ hybridization (FISH) was used to characterize structural aberration and determine breakpoint. Results 18q terminal deletion was identified with breakpoint located on a 2-Mbp region between q21.32 and q22.2 without translocation and inversion. Gene deletions were made in the critical region from 18q22.3 to q23 including myelin basic protein (MBP) and galanin receptor type Ⅰ (GALR1). Patient’s karyotype was interpreted as 46,XX,del(18)(q21.3).ish del(18)(q21.32q22.2q23)(RP11-4G81+,RP11-57F7-,qter-). Conclusions High resolution G-banding combined FISH is appropriate for the diagnosis of complicated chromosome aberration. Further research should be done to prove whether haploinsufficiency of MBP and GALR1 is the reason for growth and mental retardation or not.