Introduction. It is hypothesized that cumulative extended application of extracorporeal shockwave therapy (ESWT) can be beneficial to sustain the effect of ESWT and improve the long-term outcome in chronic diabetic foot ulcers (DFUs). Objective. The purpose of this study is to investigate the effectiveness of multiple extended booster ESWT to maintain the effects of ESWT on the outcomes of chronic DFUs. Materials and Methods. Four patients with a DFU were treated with ESWT, including 1 patient with 12 treatments and 3 patients with 6 treatments. Evaluations included clinical assessment, blood flow perfusion, and biopsy of the ulcers. Results. At 48 weeks, the results showed completely healed ulcers in 2 patients and improved ulcers in the other 2 patients. In the DFU Scale Short-Form, the score decreased in 1 patient and remained unchanged in 3 at 48 weeks. Blood perfusion increased in 1 patient but remained unchanged in 3. In immunohistochemical analysis, the angiogenic, anti-inflammatory, proliferative, and tissue repair biomarkers were elevated in 1 patient and decreased in 3. Conclusions. The effects of ESWT appear to maintain certain levels for an extended period of time at 1 year but start to show deterioration on tissue viability. Therefore, it is speculated that intermittent booster ESWT may maintain the effects of ESWT and sustain the tissue viability and repairing. The use of ESWT appeared to be effective for DFU treatment, and extended ESWT showed a tendency of sustaining ESWT effects when multiple booster treatments are utilized in patients with a DFU.