Chronic edema of the arm (postmastectomy edema, PME) is a common and incurable complication of breast cancer treatment, often developing without warning months or years later. Although the original cause of PME is damage to axillary lymph drainage routes by surgery and radiotherapy, many observations suggest that additional factors are involved. Recent attention has focused on the Starling forces in the skin and subcutis in PME. An important finding was that the protein concentration, and hence colloid osmotic pressure, of the subcutaneous interstitial fluid of the PMF arm is unexpectedly lower than in the unaffected arm, correlating negatively with the degree of swelling. There are several possible explanations for this, such as a rise in capillary filtration rate, or interstitial proteolysis. A systemic component to PME is suggested by the finding of a lower plasma protein concentration in affected women compared with a matched postmastectomy group without swelling. A recent study using intra-vital capillaroscopy has indicated that angiogenesis occurs in the skin in PME, and an increased capillary surface area for filtration could result in an increased fluid load on a compromised lymph drainage system. Further elucidation of the pathophysiological processes in PME, in particular the adjustments to Starling forces, will enable more effective therapy of this distressing condition.