Breast cancer-related lymphoedema of the arm (BCRL) results from impaired lymph drainage after axillary surgery. Little is known about lymphatic changes in the arm between surgery and oedema onset. We measured forearm muscle and subcutis lymph drainage in 36 women at 7 and 30 months after surgery by quantitative lymphoscintigraphy. None had BCRL initially but 19% had BCRL by 30 months. At 7 months muscle and subcutis drainage in both arms of BCRL-destined women exceeded that of non-BCRL women (P < 0.01). Muscle lymph drainage always exceeded subcutis drainage (P < 0.0001). Muscle lymph drainage in the ipsilateral arm was unimpaired relative to the contralateral arm. BCRL therefore developed in women with higher peripheral lymph flows. The major lymphatic load was generated by muscle; there was no pre-BCRL lymphatic impairment in the muscle of the ipsilateral arm. We propose that some women have a defined, constitutive predisposition to secondary lymphoedema. Specifically, women with higher filtration rates, and therefore higher lymph flows through the axilla that are closer to the maximum sustainable, are at greater risk of BCRL following axillary trauma, even following removal of 1-2 nodes.