Background: Changes in arm soft tissue composition, especially increased adipose tissue, has been found in advanced, non-pitting breast cancer-related lymphedema (BCRL). The aim of this study was to examine whether these changes were localized to any particular region of the arm and whether they occurred in lymphedema which still pitted to pressure. Secondary aims were to explore relationships between arm segment volumes, bioimpedance spectroscopy (BIS) measurements of extracellular fluid (ECF), and dual-energy X-ray absorptiometry (DXA) measurements of tissue composition. Methods and Results: Nine women with unilateral BCRL participated. The dominant arm was affected in 4 women, and all presented with lymphedema that pitted to pressure. Arm volume was calculated from circumferences by the truncated cone method, ECF was determined with BIS and fat and lean tissue content measured by DXA. BIS and DXA measurements for women with lymphedema were made of the whole arm and also of four 10 cm-segments measured from the ulnar styloid at the wrist. Whole arm DXA data were compared to those of 45 women of similar age and body mass index without lymphedema. All women with lymphedema had a significantly larger absolute fat mass in their affected arm compared to their unaffected arm, (median difference between arms 146.9 g). The forearm segment 10-20 cm proximal to the wrist had the highest median inter-limb fat difference of all four arm segments. Conclusions: The soft tissue composition changes associated with BCRL may occur in the presence of pitting and predominantly affect the proximal forearm.