Odontogenic keratocyst (OKC) is aggressive compared to other odontogenic cysts, but its malignant transformation is extremely rare. Odontogenic carcinoma may arise from an odontogenic cyst such as OKC. Here in, we introduce a case of odontogenic carcinoma assumed to have arisen from a preexisting OKC. Abnormal clinical and radiologic features seen in a previous or existing cyst may be warning signs of malignant transformation.