Axillary surgery for breast cancer may be followed, months to years later, by chronic arm lymphedema. A simple 'stopcock' mechanism (reduced lymph drainage from the entire limb through surviving lymphatics) does not explain many clinical aspects, including the delayed onset and selective sparing of some regions, e.g., hand. Quantitative lymphoscintigraphy reveals that lymph drainage is slowed in the subcutis, where most of the edema lies, and in the subfascial muscle compartment, which normally has much higher lymph flows than the subcutis. Although the muscle does not swell significantly, the impaired muscle drainage correlates with the severity of arm swelling, indicating a likely key role for muscle lymphatic function. A new method, lymphatic congestion lymphoscintigraphy, showed that the edema is associated with a reduced contractility of the arm lymphatics; the weaker the active lymphatic pump, the greater the swelling. Delayed lymphatic pump failure may result from chronic raised afterload, as in hypertensive cardiac failure, and may account for the delayed onset of swelling. A further novel finding is that lymph flow is raised in both the subcutis and muscle of both arms in postsurgical breast patients who later developed breast cancer-related lymphedema (BCRL), compared with patients who did not develop BCRL. This new observation indicates a predisposition to BCRL in some women. Further evidence for predisposing abnormalities is the finding of lymphatic abnormalities in the contralateral (nonswollen) arm in women with established BCRL. Such predisposing factors could explain why some women develop BCRL after sentinel node biopsy, whereas others do not after clearance surgery. Future research must focus on prospective observations made from before surgery until BCRL develops.