Jacobsen syndrome (JBS) is a haploinsufficiency syndrome caused by partial deletion of the long arm of chromosome 11. It is characterized by developmental delay (DD)/mental retardation (MR), physical growth retardation, facial dysmorphism, visceral malformations and thrombocytopenia. We report two JBS patients from China out of a total of 451 patients with unexplained DD/MR. The genotypes of these patients were compared with earlier reported patients in North America and Europe. Both patients presented with severe DD, microcephaly and facial dysmorphism; one patient had a low birth weight, congenital heart disease and structural brain abnormalities. Neither patient was thrombocytopenic at the time of diagnosis. The two deletions were 4.1 and 12.8 Mb. The 4.1 Mb deletion is the smallest of all pathogenic regions earlier reported in JBS. Therefore, the critical region underlying DD/MR might be located in the distal portion of the chromosomal segment within 4.1 Mb of the telomere. Candidate genes for DD/MR in this region include SNX19, THYN1, OPCML, NCAPD3 and NTM. One of the critical regions for craniofacial abnormalities may be within 130.3-134.4 Mb in chromosome 11q. Further analysis of Chinese JBS patients would elucidate the relation of phenotype to genotype further.