Positron emission tomography was used to measure bone marrow blood volume (BBV), an important hemodynamics parameter, in the knee. The subjects were 11 healthy male volunteers (mean age, 23.6 years; range, 21-27 years). The 15O-labelled carbon monooxide (C15O) single-breath inhalation method was used. In the distal femur, regional (r) BBV in the posterior area of the epiphysis (medial, 2. 25 ml/100 cm3 bone marrow; lateral, 2.03 ml/100 cm3) was significantly less than that in the anterior area of the epiphysis (medial, 3.48 ml/100 cm3; lateral, 3.01 ml/100 cm3) and that in the metaphysis-to-distal diaphysis (2.90-3.67 ml/100 cm3). In the proximal tibia, rBBVs in the metaphysis-to-proximal diaphysis (2. 32-2.76 ml/100 cm3) were significantly less than those in the area of the physis (medial, 3.30 ml/100 cm3; lateral, 3.53 ml/100 cm3). These regional differences in rBBV within the knee may be associated with the development of ischemic bone marrow disorders, such as steroid-induced osteonecrosis, in the knee.