There are many diseases, including aphtha, herpes, traumatic ulcers, Behget's disease, various skin diseases, sexually transmitted diseases (STD) and malignant tumors which are accompanied by persistent ulcers in the oral cavity. Many laboratory examinations are used in the diagnosis, but the most important aids are the clinical findings and the results of bacteriological examinations along with repeated histological examinations of biopsy materials from the oral cavity. It is also important to bear in mind the possibility of STDs in establishing the diagnosis.