Strong association of HLA-DRw9 in Chinese patients with recurrent oral ulcers. The frequency of HLA-DR and -DQ antigen occurrence was investigated in 80 Chinese patients with recurrent oral ulcers and 107 matched healthy control subjects of the same ethnic group. No significant differences in the presence of HLA-DQ antigens were found. However, the HLA-DRw9 antigen occurs more frequently in patients with recurrent oral ulcers (p less than 0.0001, corrected p less than 0.005) and may be considered as a genetic marker of the disease in the Chinese population. 