Capillary leak syndrome (CLS) is a serious disorder characterized by hypotension and refractory systemic oedema. CLS with marked ascites rather than systemic oedema is rare and prone to misdiagnosis and delayed treatment. We report here a case of marked ascites in an elderly male patient with hepatitis B virus reactivation. Following investigations to exclude common diseases that may have accounted for diffuse oedema and hypercoagulable state, anti-cirrhosis therapy failed and severe refractory shock developed 48 hours after admission. The patient developed mild pleural effusions followed by swelling of the face, neck, and extremities. A high cytokine concentration gradient was detected between serum and ascites. Peritoneal biopsy showed lymphoma cells. The final diagnosis was lymphoma recurrence complicated with CLS. Our case suggests that cytokine detection in serum and ascitic fluid may be helpful in the differential diagnosis of CLS. In similar cases, decisive intervention, such as, hemodiafiltration, should be implemented to lessen the likelihood of serious complications.