Purpose Given that proliferating hematopoietic stem cells are especially radiosensitive, the bone marrow is a potential organ at risk, particularly with the use of concurrent chemotherapy and radiotherapy. Existing data on bone marrow distribution have been determined from the weight and visual appearance of the marrow in cadavers. 18F-fluoro- l-deoxythymidine concentrates in bone marrow, and we used its intensity on positron emission tomography imaging to quantify the location of the proliferating bone marrow. Methods and Materials The 18F-fluoro- l-deoxythymidine positron emission/computed tomography scans performed at the Peter MacCallum Cancer Centre between 2006 and 2009 on adult cancer patients were analyzed. At a minimum, the scans included the mid-skull through the proximal femurs. A software program developed at our institution was used to calculate the percentage of administered activity in 11 separately defined bony regions. Results The study population consisted of 13 patients, 6 of whom were men. Their median age was 61 years. Of the 13 patients, 9 had lung cancer, 2 had colon cancer, and 1 each had melanoma and leiomyosarcoma; 6 had received previous, but not recent, chemotherapy. The mean percentage of proliferating bone marrow by anatomic site was 2.9% ± 2.1% at the skull, 1.9% ± 1.2% at the proximal humeri, 2.9% ± 1.3% at the sternum, 8.8% ± 4.7% at the ribs and clavicles, 3.8% ± 0.9% at the scapulas, 4.3% ± 1.6% at the cervical spine, 19.9% ± 2.6% at the thoracic spine, 16.6% ± 2.2% at the lumbar spine, 9.2% ± 2.3% at the sacrum, 25.3% ± 4.9% at the pelvis, and 4.5% ± 2.5% at the proximal femurs. Conclusion Our modern estimates of bone marrow distribution in actual cancer patients using molecular imaging of the proliferating marrow provide updated data for optimizing normal tissue sparing during external beam radiotherapy planning.