In a 62 year-old male with chronic myelogenous leukaemia, an acute phase developed with appearance of large immature cells in the bone marrow. A significant proportion of these cells was found to have engulfed autologous immature and polymorphonuclear neutrophils, erythroblasts, red cells and platelets. These abnormal cells were Philadelphia chromosome positive, and considered to be derived from the leukaemic cell line. Electron microscopic examinations revealed, in the cytoplasm of engulfed cells, no lysosomes or phagosomes which are typically seen in phagocytic cells, nor any evidence of degenerative changes in either engulfing or engulfed cells. These findings suggest that this phenomenon be considered as emperipolesis rather than phagocytosis. The pathophysiological mechanism of this phenomenon is uncertain.