Background and purpose: Oral lichen planus is a chronic inflammatory disease of the oral mucosa. The exact cause is not yet well understood. Possible associations have been suggested between blood groups and susceptibility to various diseases. This study investigated the distribution of ABO blood groups and Rh system in patients with oral lichen planus in Iran.

Materials and methods: In a cross-sectional study, 100 patients with oral lichen planus attending Oral Medicine Department in Shahid Beheshti dental school, Tehran, were examined. ABO blood groups and Rh type were recorded. Data was analyzed in SPSS V.18 applying Chi-square test.

Results: Among 100 patients, 38 (38%) had blood group O and others had blood group A (28%), B (18%), and AB (16%), respectively. Also, 96 patients were Rh-positive while four were Rh-negative.

Conclusion: Patients with oral lichen planus in this study had higher distribution of O blood group than other blood groups. Also, most of them were Rh-positive.