The clinical and pathological features of histiocytic medullary reticulosis (HMR) are described in five patients. Treatment by splenectomy and the MOPP routine resulted in a transient improvement in platelet and neutrophil counts and resolution of anaemia and apparent prolongation of survival in two of the five patients. The relationship of HMR to other histiocytic disorders is discussed. HMR is defined as a unique malignant histiocytosis in which malignant histiocytes engage in phagocytosis of eryrhrocytes, leucocytes and platelets.