The use of fine-needle aspiration biopsy (FNAB) in the diagnosis of odontogenic tumors seems to have attracted little attention. The presence of a firm preoperative diagnosis helps preventing suboptimal surgery, contributing to avoid recurrence of these tumors. A case of ameloblastic carcinoma of the mandible diagnosed by FNAB is presented in this report, illustrating its effectiveness for preoperative diagnosis of odontogenic tumors. A 74-year-old female presented with a painless swelling in the right mandibular angle. A panoramic radiograph revealed a radiolucent lesion in the body of the mandible. Cytological smears from FNAB in the area revealed basaloid cells with a palisade arrangement and presence of stellate-shaped cells. These cytological features lead to the diagnosis of ameloblastoma. However, when there are atypical cells and atypical mitoses, as in the present case, diagnosis of ameloblastic carcinoma may be established. The patient underwent chemotherapy, showing remission of the lesion after treatment. FNAB is a minimally invasive, safe, fast and inexpensive method for diagnosing benign and malignant ameloblastomas, which ensures that patients have a proper treatment without the need of performing an incisional biopsy, especially in neoplastic cases.