Severe post-thrombotic syndrome (PTS) is responsible for considerable disability, reduced quality of life and increased health care costs. Current therapies are limited and often ineffective. We performed a two-centre, randomized, cross-over controlled trial to evaluate Venowave, a novel lower-limb venous-return assist device, for the treatment of severe PTS. Eligible subjects were allocated to receive, in randomized order, Venowave for eight weeks and a control device for eight weeks. The eight-week treatment periods were separated by a four-week period when no device was used (i.e. wash-out period). The primary outcome measure was a 'clinical success' defined as: i) reported benefit from the device; and ii) moderate or greater improvement in symptoms of PTS; and iii) willingness to continue using the device. Secondary outcome measures included quality of life (QOL) as measured by VEINES-QOL questionnaire (higher scores indicate better QOL), and PTS severity as measured by the Villalta PTS scale (higher scores indicate more severe PTS). The study was registered with Clinical Trials. gov (NCT00182208). Thirty-two patients were enrolled. Of these, 26 (80%) were also using graduated compression stockings. Twenty-six participants completed both trial periods. Clinical success occurred in 10 (31%) participants receiving Venowave and four (13%) participants receiving the control device, with two (6%) participants reporting a clinical success with both devices (P = 0.11). Mean VEINES-QOL score at the end of study period was significantly greater (P = 0.004) for Venowave (52.5; SD 5.8) compared to control (50.2; SD 6.2). Mean Villalta scale score at the end of study period was significantly decreased (P = 0.004) for Venowave (12.2; SD 6.3) compared to control (15.0; SD 6.1). In conclusion, Venowave appears to be a very promising new therapy for patients with severe PTS, which may be used alone or in combination with graduated compression stockings.