OBJECTIVE To ascertain the karyotype of a girl with moderate mental retardation and growth retardation, perform correlation analysis between chromosomal variation and phenotype, and investigate the application and superiority of array-based comparative genomic hybridization (array-CGH) in clinical cytogenetic diagnosis. METHODS G-banded chromosome analysis, array-CGH, fluorescence in situ hybridization (FISH) and real-time quantitative PCR (RQ-PCR) were used to ascertain the karyotype of the patient and her relatives. RESULTS G-banding analysis of the patient showed a derivative chromosome 10 with an extra fragment on its long arm terminal, both her father and grandmother had an apparently balanced translocation t(4;10)(q25;q26). Array-CGH revealed that the breakpoint on chromosome 4 was located at 4q26. In addition, a microdeletion of about 0.54 Mb del(10)(q26.3) was identified from the patient. FISH and RQ-PCR confirmed that the del(10)(q26.3) was also present in both her father and grandmother. CONCLUSIONS No recognizable phenotype was associated with del(10)(q26.3). The abnormal phenotypes presented in the patient may be ascribed to the 4q26-q35.2 triplication. Further more, compared with conventional cytogenetic analysis, array-CGH is of high resolution and high accuracy.