Summary.A survey of the Western literature of histiocytic medullary reticulosis (malignant histiocytosis) provided 104 case reports with sufficient data from which the presence or absence of an absolute lymphopenia could be ascertained. Of these, 46 (44%) were lymphopenic at presentation or within the subsequent 10 days. A survey of a series of publications containing detailed peripheral leucocyte counts recorded in patients with diseases that, like HMR, may present with, or develop, pancytopenia showed that the incidence of lymphopenia ranged from 14% (Hodgkin's: stages I and II) to 46% (acute myelofibrosis; systemic lupus erythematosus; angio‐immunoblastic lymphadenopathy). It was concludedathat HMR should be added to the list of accepted causes of lymphopenia,bthat lymphopenia, as an aid to the diagnosis of HMR, will be of limited value,cthat when the peripheral leucocyte count of a patient is recorded in a case report, it should be accompanied by a full differential coun