A multi-centre, double blind randomized clinical trial was designed to assess the efficacy and safety of orally administered 0-(beta-hydroxyethyl)-rutosides (HR) capsules in the treatment of 101 patients with post-thrombotic syndrome. Seventeen patients were excluded from the analysis for violation of the study protocol, 41 received HR capsules (1,200 mg/day) and 43 placebo. Mean follow-up scores at the 4th and 8th week show that the HR patients displayed an improved state of tiredness as compared to the placebo's. The mean circumference of the calf for the HR group decreased from 390 (+/- 33) mm at visit one to 382 (+/- 33) mm at visit three, with a mean circumference reduction of 8.7 (+/- 8) mm, compared to a steady placebo circumference of 387 (+/- 31) mm at all 3 visits with a mean circumference reduction of only 2 mm (+/- 9). The estimated treatment effect at week 8 was -6.7 mm, 95% confidence interval (-10.3, -3.0). The mean circumference of the ankle, decreased from 243 (+/- 20) mm to 238 (+/- 20) mm at the 4th week, contrasted with a constant placebo circumference of 241 (+/- 22) mm at both visits. The estimated treatment effect at week 4 was -5.4 mm, 95% confidence interval (-10.2, -0.6). However, at week 8, the estimated treatment effect was only -3.4 mm, 95% confidence interval (-8.6, +1.8). In conclusion, HR capsules may show an improvement in the clinical symptoms and may show a mean circumference reduction of the calf and ankle at the 8th week, in patients with post-thrombotic syndrome.