To assess the activity of a purified, micronized, flavonoidic fraction (Dios; Daflon 500 mg*) on upper limb lymphedema occurring after breast cancer therapy, a monocenter, randomized, double-blind, parallel group vs placebo (Plac) trial was carried out. One hundred and four women with lymphedema were included; 94 completed the study (46 Dios, 48 Plac). A subset of 24 patients with more severe lymphedema (10 Dios, 14 Plac) was subjected to a separate analysis. Treatment consisting of Dios or Plac was given two tablets daily over a six-month period. A radionuclide lymphoscintigraphy using technetium-99m was performed at inclusion and at the end of the treatment. The upper limb volume was measured every two months. In the overall population the evolution of parameters was not different between Dios and Plac. In the 24 patients with a more severe lymphedema, the lymphoscintigraphic parameters (m +/- sd) were as follows: lymphatic migration speed was significantly improved by Dios in comparison with Plac (delta Speed cm/minute: 0.84 +/- 0.6 vs 0.14 +/- 0.26, P = 0.005). The half-life of the colloidal compound was significantly improved over time in the Dios group (delta half-life = 10.3 +/- 13.07 minute, P = 0.034) but not in the Plac group (delta half-life = 0.53 +/- 15.51 minute, P = 0.086). The change over time of colloidal clearance was close to significance in the Dios group (delta clearance microL/minute: 2.18 +/- 3.10, P = 0.054) but not in the Plac group (0.11 +/- 2.26, P = 0.86). No significant difference was found for evolution of lymphedema volume, despite a tendency in favor of Dios. This can be related to wide distribution of volume values and small numbers of patients. In conclusion, these results suggest a beneficial therapeutic activity of Dios at the usual dose of two tablets/day in patients affected with more severe lymphedema. The clear improvement of the lymphatic speed illustrates its known lymphokinetic activity. Further studies with a higher dosage could confirm the beneficial activity of this drug in secondary lymphedema.