Among Japanese, ossification of the posterior longitudinal ligament of the spine (OPLL) is a leading cause of myelopathy, showing ectopic bone formation in the paravertebral ligament. We have provided genetic evidence that the collagen alpha2 (XI) (COL11A2) locus of chromosome 6 constitutes susceptibility for OPLL. Five distinct single nucleotide polymorphisms (SNPs), identified in COL11A2, were combined to construct possible haplotypes by the use of a maximum likelihood program. Estimated haplotype frequency was compared in OPLL patients and non-OPLL controls. We report a gender-specific association of the COL11AA2 haplotvpe with OPLL. The frequency of the most commonly observed haplotype was significantly higher in male patients (P = 0.0003) compared with controls, but not in female patients (P = 0.21). OPLL is predominantly observed in males. with a prevalence ratio of 2:1, and our gender-specific associations indicate that genetic factors involving COL11A2 play a specific role in the etiology of OPLL exclusively in males.