16 cases of calcifying odontogenic cysts (C.O.C.) were studied and reevaluated. It could be concluded, that the group contained two entities, a cyst and a neoplasm. The cyst occurs as three variants. 1. A simple unilocular cyst with moderate mural proliferations of epithelium and no, or sparse amounts of, dentinoid (dysplastic dentin); it seems to occur during man's entire life span. 2. A unilocular cyst which produces compound or complex odontomes in its luminal part, more rarely it may instead produce an intramurally growing ameloblastic fibroma, which may call for more radical surgery. It occurs mainly in patients between 10 and 29 years of age. 3. A unilocular cyst with extensive luminal as well as mural ameloblastomalike proliferations of epithelium. The C.O.C. may be located outside or inside the bone according to the location of the source of odontogenic epithelium, from which it develops.The neoplasm shows an entirely different structure. It consists of ameloblastoma-like strands and islands of odontogenic epithelium growing infiltratively in a mature connective tissue. Varying amounts of ghost cells are seen in the epithelium and varying amounts of dentinoid is formed in contact with the odontogenic epithelium. The term«Dentinogenic ghost cell tumour»is suggested for this lesion. It is possible that it occurs predominantly in the later part of life. It occurs as an extraosseous as well as an intraosseous lesion. Recurrence has been observed following cystectomy.