Calcifying odontogenic cyst (COC) is an uncommon developmental odontogenic lesion that demonstrates considerable histopathologic diversity and variable clinical behavior. Objectives: To assess the proliferative activity in 4 different types of COC. Methods: Twenty cases COC (4 non-proliferative type, 8 proliferative type, 6 odontoma-associated type and 2 ameloblastomatous type) diagnosed at the Department of Oral Pathology, Faculty of Dentistry, Chulalongkorn University were stained with antibody to proliferative cell nuclear antigen (PCNA). PCNA labeling index was calculated as percentage of positively stained cells. Data obtained were evaluated by descriptive statistics. Results: Ameloblastomatous type of COC elicited the highest PCNA labeling index (Mean±SE=38.9±3.10) followed in descending order by proliferative type (Mean±SE=33.79±4.56), non-proliferative type (Mean±SE=13.43±5.98) and odontoma-associated type (Mean±SE=7.76±3.07). Conclusions: Different types of COC show different PCNA labeling index. PCNA labeling index of different types of COC correspond well with histopathologic feature and clinical behavior. This research project was supported by Dental Research Fund, Dental research project 3205-312#09/2003 Faculty of Dentistry, Chulalongkorn University.