Provided certain methodological precautions are observed, mercury strain gauge venous occlusion plethysmography represents one of the best available function tests in phlebology to evaluate the efficacy of a phlebotonic agent. All physiological and environmental parameters liable to affect venous tone must be determined and controlled before and during the procedure, which is to be carried out in controlled and fully reproducible conditions. Intra and inter-observed reproducibility of results must be checked regularly in each laboratory. The objective character of the parameters assessed (Hmax, delta Vmax, T50, T2p...) does not spare the need for randomized, crossover or comparative, double-blind placebo-controlled trials, as plethysmographic results may show wide variations from one subject to another, and even from one moment to another, in chronic venous insufficiency, depending on the subject's environment and occupation. Furthermore, knowledge of treatment by the observer may affect the validity of the results. Taking into account all of these methodological pre-requisites, the phlebotonic activity of a single dose of Daflon 500 mg in lower limb chronic venous insufficiency was assessed in placebo-controlled clinical pharmacological studies. The following characteristics emerged: (1) significant improvement of venous capacitance, distensibility, and emptying times, as from the 2nd hour following Daflon 500 mg administration; (2) dose-response relationship, with an optimal dose of 2 tablets per day; (3) rapid onset and long duration of action; (4) significant hemodynamic effect on the venous system, whatever the type of venous insufficiency: organic, functional or gestational.