Sections of tissue embedded in paraffin wax from 18 selected odontogenic cysts were studied both histologically and immunohistochemically with antibodies to cytokeratins using the indirect peroxidase technique. The cysts were divided on a clinical and histological basis into two equal groups comprising dentigerous cysts and odontogenic keratocysts. It was possible to differentiate the two cyst types in every case by the pattern of staining using the monoclonal antibody LP34 for cytokeratins of intermediate molecular weight. The monoclonal antibody CAM 5.2 for cytokeratins of low molecular weight was not discriminatory. Such a clear distinction may prove useful diagnostically in distinguishing between two cysts of similar appearance but very different behaviour.