Follow-up examinations of 670 patients with oral leukoplakia during a 30-year-period showed cancer development in 40 cases, i.e. 6%. Dysplasia was observed in 24% of the histologically examined leukoplakia cases; 13% of the dysplasia cases subsequently showed development of carcinoma. The age distribution revealed the prevalence of leukoplakia in the age-group 51-60 years; that of carcinoma in the age-group of 61-70 years. The sex distribution showed a male-female ratio of 3.2 : 1 in the leukoplakia-group, and a 1.9 : 1 ratio in the carcinoma-group. The tongue and the lips were the site of predilection for malignant transformation and for dysplasia. Among aetiological factors, Candida albicans infection and the simultaneous existence of several aetiological factors seemed to play a role in malignant transformation. Erosive leukoplakia showed the highest risk, developing in 25.9% of the cases into cancer.