An electron and optical microscopic study of juxtaposed odontogenic keratocyst and carcinoma. Odontogenic keratocyst and squamous cell carcinoma commonly occur within the oral cavity; however, the juxtaposition of these lesions is rare. The light microscopic and ultrastructural features of such an event are reported. Although some morphologic similarities between the cyst and tumor were observed, definitive evidence of a common origin was not obtained. 