Objective To compare the features and diversity of the immunoglobulin heavy chain variable region (IgH) gene in neonates and adults. Methods DNA was extracted from the cord blood of 10 full term neonates and from the peripheral blood of 8 adults. The IgH gene was amplified using the nested PCR technique, and then cloned and sequenced. Results The usage of variable (VH) and diversity (D) segments in neonates was similar to that in adults: V H3 and D2 usage were preferred, while V H7 was incidentally used. With respect to the usage of joining segments (J H), there was a bias towards J H3 in neonates, while there was a tendency to randomize in adults. N region inserts at the V H-D-J H point of rearranged IgH existed in both neonates and adults. The N region-D gene-N region (NDN) length in neonates was ( 21.2 ± 5.7 ) base pairs (bp), which was shorter than that in adults [( 26.6 ± 6.5 ) bp]. There was no base loss in the neonatal IgH gene, whereas there was some loss in adults. Neonates had a much lower somatic mutation rate ( 29.2% ) than adults ( 77.1% ). Open reading frames (ORF) occurred in the rearranged IgH gene of neonates and adults at the ratio of 91.7% and 100%, respectively. Conclusions Differences in IgH gene rearrangement exist between neonates and adults. Rearranged IgH of neonates is in an activated condition, but its diversity is limited secondary to preferential selection of VH, D and JH segments, short NDN and decreased somatic mutation rate.[