It has been a frequent observation that many patients with leukemia and multiple myeloma bleed freely through the marrow needle at the time of bone marrow aspiration although hemorrhagic tendencies may not be present clinically. In addition , throbbing bone pain and bone tenderness are not infrequently observed in these patients (1). Such clinical observations suggest that some degree of pressure elevation may be present in the marrow cavity in these diseases. It was believed of interest to determine the pressure relationships within the human marrow cavity under a variety of physiologic stimuli. MATERIALS AND METHODS The marrow pressures of 14 patients with neoplastic diseases having essentially normal marrows, as determined by microscopic examination, and of 10 patients with leukemia and one patient with multiple myeloma were determined by means of recording strain gauges attached to clinical bone marrow needles. The patients were studied under resting but not basal conditions, without previous sedation. The pressure in both the sternal and iliac crest marrows was measured in most patients, either simultaneously or serially. The sternal site was at the level of the second interspace, and the iliac crest at a site two centimeters posterior to the anterior superior iliac spine. The usual precautions of sterility and local procaine anesthesia of skin and periosteum were employed. A bonded SR4 wire resistance strain gauge and recording apparatus developed for clinical pressure measurements by Bierman and Jenkins (2) were employed for pressure determinations. The gauge was connected directly to the bone marrow needle after the needle and gauge chamber had been filled with heparin-saline solution. The saline within the manometer was equalized with the ambient pressure, after which the chamber was sealed. The output of the strain gauge was connected to a strain indicator and photographically recorded with a multiple channel recording oscilloscope. The Turkel-Bethell bone marrow needle employed in these studies had a 14-gauge bore and was equipped with a trephine stylette which made it useful for removing bony spicules which frequently entered the needle during its implantation. After implantation, the needle was filled with heparin-saline solution, care being exercised to exclude air bubbles. It was found necessary to premedicate the patients with 50 to 100 mg. of heparin intravenously shortly before the studies to prevent clotting within the needle in the marrow. Smaller amounts of heparin were used in patients with leukemia, or completely omitted where manifest bleeding tendencies existed. A modified strain gauge strapped over the lower chest cage was utilized as a pneumotachometer, and a single lead EKG tracing was made in each patient for pulse de-terminations. The marow pressure, respiratory rate and amplitude, and EKG record were simultaneously recorded photographically. The marrow pressures were studied with the patients at rest in supine position, during exaggerated respiration, coughing, Valsalva and Mueller...