The frequency of secondary lymphedema occurrence following breast cancer therapy has been decreasing since new conservative surgical treatment and radiotherapy are applied. However, about 20% of female patients treated for breast cancer will develop mild swelling of the arm or forearm and seek specific treatment. Decongestive physiotherapy remains the most effective treatment, although medical treatment is often necessary. Coumarin, tested both in vitro and in vivo, has yielded true stimulation of lymphatic function. Like other benzopyrone derivatives, coumarin acts through macrophage stimulation. A randomized double blind study versus placebo has confirmed the occurrence of significant improvement of these mild swelling cases, and recommend the use of coumarin as supplemental therapy in mild lymphedema.