To determine the interrelation between ABO blood types of denture wearers, denture plaque accumulation, and denture stomatitis, 442 denture wearers were studied using a simplified culture method, which is convenient to use in the dental office. The degree of plaque accumulation and the occurrence of denture stomatitis varied depending on the blood type of the patients. Especially in blood group O compared with other types, both denture plaque accumulation and denture stomatitis were found to be higher or more severe. These results suggest that the ABO blood group is one of the etiologic factors of denture stomatitis and that denture wearers of blood group O are more susceptible to denture stomatitis.