A 3-year-old girl with an inherited translocation of 4q onto the long arm of chromosome 10 is reported as a case of 'pure' partial trisomy 4q. The main clinical features of the patient included moderate mental retardation (IQ = 40), growth retardation, language handicap, right hand preaxial hexadactyly, and abnormal electroencephalogram. Conventional cytogenetic analysis showed partial trisomy 4q25-->qter was present in the patient, both patient's father and grandmother were apparently balanced translocation t(4;10) (q25;q26) carriers, which suggests that the partial trisomy 4q25-->qter in the patient results from unbalanced segregation of the father's translocation. The extension of the trisomic 4q region was redefined by array comparative genomic hybridization (array-CGH), the breakpoint on chromosome 4 was located at 4q26 (between 115,596,658 bp and 118,785,802 bp from the telomere of 4p), and an approximately 0.54-Mb microdeletion del(10)(q26.3) (between 134,750,859 bp and 135,286,223 bp from the telomere of 10p) was identified. FISH and RT-qPCR confirmed that del(10)(q26.3) was also present in both father and grandmother, which suggests that their translocations are actually unbalanced and the patient's abnormal phenotypes may only ascribe to partial trisomy 4q26-->q35.2.