Idiopathic thrombocytopenic purpura (ITP) is a disease putatively relating to abnormal immune function and auto-antiplatelet immunoglobulin. We examined whether polymorphism of CD72, an inhibitory receptor of B cells, affect the susceptibility to ITP, or associated with the clinical characteristics of ITP. A case-control study was carried out in 206 Chinese ITP patients and 169 healthy controls. The detection of variable number of tandem repeats in CD72 intron 8 was performed by polymerase chain reaction and subsequent analysis with polyacrylamide gel electrophoresis. We did not find direct association between CD72 genotypes and susceptibility to ITP. The haplotype that contained one repeat of 13 nucleotides in intron 8 (designated as *1, and haplotype containing two repeat of 13 nucleotides in intron 8 is designated as *2) was significantly associated with early first onset age (< or = 14) in ITP patients (P = 0.03). ITP patients with CD72*1\*1 and *1\*2 genotype had a 3.09-fold [95% confidence interval (CI), 1.32-7.25] and 1.98-fold (95% CI, 0.92-4.25) increased risk of appearing ITP manifestation at their childhood respectively. The haplotype CD72*1 is apparently a risk allele, whereas CD72*2 a protective allele for child-onset of ITP disease.