A series of 20 consecutive patients with an osteosarcoma of the jaws has been evaluated with regard to possible professional diagnostic delay. When set at an arbitrarily chosen period beyond three months, professional delay occurred in 15 patients, the mean being 21 months and the median 11 months. In five of the 15 patients a wrong diagnosis has been rendered on the biopsy specimen, being fibrous dysplasia (2x), osteoma (2x) and, in case of pala-tomaxillary swelling, pleomorphic adenoma (1x). In the other ten patients the initial clinicoradiographic features were misleading and apparently not indicative of a malignancy, except for one patient in whom a distinct widening of the periodontal ligament, as expressed on a periapical film, has been overlooked or not properly interpreted. It has not been possible to assess the possible influence of the delayed diagnosis on the prognosis.