Neutrophil marrow egress was examined in 6 polycythemia vera patients using 3H-thymidine (3H-TDR) pulse labeling. The granulocytopoietic proliferation activity ranged between moderate depression (1420 neutrophils per mul blood) and marked enhancement (22470 neutrophils per mul blood). Egress characteristics did not significantly deviate from the ones found in subjects with normal granulocytopoietic proliferation activity. Shortest cell sojourn in the medullar nonproliferating granulocytopoietic pool before cell transit into the blood, i.e. "neutrophil emergence time", ranged between 67-102 h (median 76 h). After this time interval labeled band and segmented neutrophils simultaneously appeared in the peripheral blood. However, during the early influx phase of labeled neutrophils labeling indices of band forms increased more rapidly than that of segmented forms. In segmented forms the initial curve increment followed approximately an exponential function. Labeling indices doubled in 10-13.5 h with remarkably small variance in the different disease states examined. The results indicate that the main factors influencing neutrophil marrow egress, i.e. maturation of granulocytopoietic cells, structure of marrow sinusoids and control mechanisms, function normally in polycythemia vera.