The aim of this retrospective study was to determine the frequency and malignant transformation rate of oral lichen planus and leukoplakia in a large group of oral medicine patients. Study included 12 508 patients who were referred between 1998 and 2007 to the Department of Oral Medicine. The frequency of OLP was 4.30%, leukoplakia 1.11%, and combined diagnoses 0.14%. In primary biopsies dysplasia was found in 12.96% of patients with leukoplakia and not in one with OLP and combined lesions. The highest frequency of leukoplakia was found in smokers. Women were found as predominant sufferers of both diseases and their combination. During the observed period often years malignant transformation of OLP was not detected, unlike leukoplakia where it was 0.64%. The frequency of OLP and leukoplakia in our study are comparable to other similar studies. The highest frequency of malignant transformation was observed in those patients who did not respond to our invitation to regular check-up. It is therefore neccessary to perform a detailed examination of the oral cavity in these patients and to raise patients awareness of the disease and the importance of regular follow-up.