The identification of the proliferative activity in tumours may be useful to predict the biological behaviour of different lesions. Proliferating cell nuclear antigen (PCNA) has been used for the evaluation of the proliferative ability of many lesions. In this study 22 ameloblastomas (4 follicular, 5 plexiform, 4 acanthomatous, 5 unicystic, 4 recurrent), 12 odontogenic keratocysts (OKC), 8 dentigerous cysts (DC), and 12 radicular cysts (RC) were analysed. PCNA+ cells were present in all cyst types but the OKC contained the highest number of PCNA+ cells. In OKC the location of PCNA+ cells was mainly suprabasal. In ameloblastoma PCNA+ cells were located mainly in the peripheral portion of the tumour islands. Statistical analysis showed that ameloblastoma had higher PCNA+ cell counts than OKC (P < 0.0001); OKC had higher values than DC and RC (P < 0.0001). Recurrent ameloblastoma presented higher PCNA+ cell counts than other types of ameloblastoma, while unicystic ameloblastoma showed lower values than acanthomatous, plexiform and follicular ameloblastomas (in this latter case the difference was not statistically significant). These data could help to explain the different biological behaviour of these lesions.