The objective of the present study was to investigate the effects of a purified micronized flavonoid fraction (Daflon 500 mg) on the increased capillary filtration of albumin (CFA) which is an early change in the microcirculation of patients with diabetes. A placebo controlled trial lasting 30 to 42 days including two equal groups of 20 patients, following a 15-day initial placebo run-in phase. The CFA test employed consisted of injecting albumin labelled with technetium (99mTc) intravenously, applying a venous tourniquet to one arm, and counting the radioactivity on the ipselateral forearm using an external gamma camera. The disappearance of radioactivity was analysed by means of two parameters after the tourniquet had been released, i.e. albumin retention (AR) and the LF/HF index which indicates lymphatic function in interstitial protein clearance. All patients included exhibited an increase in these two indices before the initial placebo run-in phase and at the end of the run-in period. At the end of the treatment phase, AR had significantly decreased in the group treated by the flavonoid fraction (FF) by comparison with the placebo group, and had normalized in 65% of patients in the FF group, compared with 25% of patients in the placebo group (p = 0.010). The LF/HF index normalized in 55% of cases in the FF group, compared with no patients on placebo (p = 0.00001). This study suggests that the flavonoid fraction tested here can improve and even normalize capillary filtration of albumin in diabetic patients.