Objective We compared the difference of diagnosing macrosomia using the body mass index (BMI)and body mass,so as to investigate whether BMI play an important role in the diagnosis and management of macrosomia in our clinical work.Methods We analysed 5522 newborns (without any maternal complication)delivered in Shengjing Hospital of China Medical University from Jan.2004 to Apr.2009,all of them were full term,singleton and with the birth body mass larger than 2500 g,among them 4989 were in the group with body mass <4000 g,that was 2510-4000 g.533 cases were in the group of body mass ≥4000 g.By both body mass and length,we got the BMI.According to statistical receiver operating characteristic curve(ROC),we determined the cutoff of BMI for diagnosing macrosomia,in addition the sensitivity and specificity of it. Using this newly gotten BMI cutoff as a method to diagnose macrosomia and analyse the results.Results (1)When the newborns with birth length 40-43 cm.the mean birth body mass was(3010 ±351)g,BMI was(17.0 ±2.7)kg/m2;the newborns with birth length 48-51 cm,the mean birth body mass was(3450 ±313)g,BMI was(13.2±1.4)kg/m2;newborns with birth length 56-60 cm,the mean birth body mass was(4332 ±456)g,BMI was(12.5 ±1.3)kg/m2,The longer the birth length,the larger the birth body mass,while the less BMI.(2)Determined by ROC curve,the BMI value could be used to diagnose macrosomia was 14.2 kg/m2.with sensitivity of 78.4% and specificity of 85.0%, the area of under curve was 0. 892. (3) By the BMI cutoff ( 14. 2 kg/m2 ), 111 macorsomia with birth body mass ≥4000 g were not macrosomia any more (20. 8%, 111/533 ),422 still were macrosomia (79.2% ,422/533) ; while for those birth body mass <4000 g, 728 were macrosomia determined by this BMI cutoff ( 14. 59%, 728/4989 ), 4261 were still not macrosomia ( 85.41%, 4261/4989 ). Using BMI cutoff 14. 2 kg/m2 to diagnose macrosomia, within the group of birth body mass ≥4000 g, their birth length in macrosomia and non macrosomia was (52. 2 ± 1.8) cm and ( 55.6 ± 1.3 ) cm respectively, the difference was significant (P <0. 01 ) ;while within the group with body mass <4000 g, the birth length of macrosomia and non-macrosomia was (49.0 ±2. 2) cm and (50. 8 ±2. 2) cm respectively,the difference was significant as well (P <0. 01 ). The whole incidence of macrosomia was 20. 83% (1150/5522) determined by this BMI cutoff. Conclusions Birth body mass and BMI in determining macrosomia show some bias, and birth length relates with this difference, which suggests birth length maybe play an important role in determine the macrosomia. We suggest it is very necessary to use BMI≥ 14. 2 kg/m2 in the diagnosis and management of macrosomia.