The stereologic characteristics of monocytes from patients with acute nonlymphocytic leukemia containing a monocytic component (FAB M4 and M5), and the monocytes from normal individuals were determined by morphometric analysis. The cells studied were monocytic cells beyond the promonocyte stage of development, as defined by ultrastructural criteria. Parameters evaluated included cell and nuclear volumes and surface areas, mitochondrial and myeloperoxidase (MPO)-positive secretory granule volume and numerical density as well as volume and number of the organelles/cell. Peripheral blood and bone marrow monocytes of leukemic patients could not be distinguished by their cell or organelle stereologic characteristics. Monocytes from patients with both M4 and M5 acute leukemia had relatively large cell and nuclear volumes. Mitochondrial volume density and volume/cell were also high in monocytes from leukemic patients (M4; 21 microns 3/cell, M5 20 microns 3/cell) as compared with monocytes from normal individuals (8.5 microns 3/cell). On the other hand, MPO-positive secretory granule stereologic parameters in monocytes from leukemic patients were indistinguishable from those of normal individuals. A small number (3 of 18) patients showed very low monocyte MPO-positive granule volume densities. There was a slight positive correlation between MPO-positive granule volume density and patient survival time. No relationship between mitochondrial characteristics and survival was noted.