Background <p>During hemodialysis, the formation of thrombi inside the venous air trap chamber is associated with high venous pressure. Direct observation inside the venous air trap chamber involves invasive procedures and risks affecting the closed-circuit system. Methods for non-invasive observation inside venous air trap chambers have not yet been generalized.</p> Case presentation <p>A thrombus was identified in the filter inside a venous air trap chamber during emergency hemodialysis performed on a man in his 60s with acute renal dysfunction. During subsequent hemodialysis sessions, we regularly monitored the venous air trap chamber using ultrasound imaging and employed colour Doppler to identify the thrombus. Images were also captured that revealed signs of anomalies.</p> Conclusions <p>Ultrasound imaging allows non-invasive observation of the interior of a venous air trap chamber. Employing this non-invasive method poses less risk to patients and can potentially be applicable to continuous renal replacement therapy in circuit configurations that use chambers.</p>