Two cases of osteoclast-type giant cell tumour of the pancreas (OGTP) are presented and compared with similar tumours of other locations and pancreatic carcinomas. One of the tumours was analyzed by immunohistochemical methods. The mononuclear stromal cells and osteoclast-like giant cells, which characterize this very rare neoplasm, reacted with an antibody against vimentin, but were not decorated by antibodies against lysozyme, alpha-1-ACHT, alpha-1-AT. Pleomorphic mononuclear cells in osteoid additionally contained osteonectin and could thus be identified as osteoblasts. Only the tumour glands stained positively with panepithelial keratin antibodies and antibodies against the keratin polypeptides 7, 18, 19. These results demonstrate for the first time the mesenchymal differentiation of the OGTP, which in some cases is also able to form epithelial structures. The immunohistochemical reactions and the characteristic morphology of the tumour show the OGTP to be an entity which must be differentiated from pancreatic carcinoma, especially from its giant cellular subtype.