The term of "lethal midline granuloma" has been used to apply to a number of diseases. In this paper, it is argued that this term should be only a clinical description of a relentlessly progressive destructive lesion of the nose and face. A list of all the possible causes is presented. There is a group of patients who do not correspond to any specific etiology. Reviewing the observations in the literature and our own, we think that such cases are best classified according to their histological appearance. Those without atypical cells (20% of cases) are always localized to the upper respiratory tract, while those with atypical cells (histiocytes?), 80% of cases, may become generalized and/or may evolve to a malignant lymphoma. When localized, both forms (with and without atypical cells) respond very well to high-dose radiation therapy (40 to 60 Gy). In disseminated forms or relapses following radiotherapy treatment, chemotherapy, the only available alternative treatment, has met with little success. We present the clinical and pathological features of six such patients, of whom one had no atypical cells. Radiotherapy produced a clinical remission in five of five cases, but of shorter duration than that reported in the literature. No treatment other than radiotherapy has yet proven to be regularly efficient in this rare and mutilating disease.