Objective To evaluate the effects of extracorporeal shockwave therapy (ESWT) on pain, function, and ultrasonographic features of chronic stroke patients with knee osteoarthritis (OA). Methods A total of 18 chronic stroke patients (33 knee joints) with unilateral or bilateral knee OA (Kellgren-Lawrence grade >= 1) were enrolled in this study. The patients were randomly allocated to an experimental group receiving ESWT (n=9) or a control group receiving sham ESWT (n=9). For the ESWT group, patients received 1,000 pulses weekly for 3 weeks, totaling to an energy dose of 0.05 mJ/ mm(2) on the proximal medial tibia of the affected knee. The assessments were performed before the treatment, immediately after the first treatment, and 1 week after the last treatment using the following: the visual analog scale (VAS) for pain; patient perception of the clinical severity of OA; the Korean version of Modified Barthel Index (ambulation and chair/bed transfer); the Functional Independence Measure scale (FIM; bed/chair/wheelchair transfer, toilet transfer, walking, and stairs); and ultrasonographic features (articular cartilage thickness, Doppler activity, and joint effusion height). Results The experimental group showed a significant improvement in VAS score (4.50 +/- 1.87 to 2.71 +/- 1.38) and patient perception of the clinical severity of OA (1.87 +/- 0.83 to 2.75 +/- 0.46). The bed/chair/wheelchair transfer components of the FIM score also improved significantly (4.12 +/- 1.55 to 4.62 +/- 1.30). In terms of the ultrasonographic features, increased Doppler activity was observed in the medial knee in the experimental group immediately following ESWT. Conclusion It is suggested that ESWT may reduce pain and improve function in chronic stroke patients with OA, and may increase vascular activity at the target site.