During neurosurgical operations of 73 patients alterations of the leptomeninx were found additionally other clinical diagnoses or confirming the clinical diagnosis of a chronic leptomeningopathia. In these cases tissue was taken for histological examination. In 65 cases there were alterations of all parts of the leptomeninx, mostly connected with inflammatory infiltrations. The commonly used terms "arachnitis" or "arachnoiditis" for these circumscribed chronic leptomeningopathias are not to be accepted because of different reasons. Instead of these names a describing diagnosis is recommended.