BACKGROUND: Most cases of thoracic injury are the result of minor trauma, treated out of hospital. Approximately 10 % of patients with minor thoracic trauma develop delayed haemothorax, with risk of fibrosis and empyema. Traumatic diaphragmatic injury following blunt trauma can be difficult to diagnose, and is a rare condition associated with high-energy trauma. CASE PRESENTATION: A 63-year-old man with multiple rib fractures following a traffic accident was readmitted for chest tube drainage 19 days after trauma, due to delayed haemothorax. Four days later, the patient developed massive haemothorax due to pleural fibrinolysis. Video-assisted thoracic surgery confirmed lacerated diaphragm with protruding, bleeding omentum. INTERPRETATION: This case report describes an atypical presentation of a common complication following thoracic trauma. Identification and control of patients at risk of developing delayed haemothorax is of clinical importance to reduce the risk of long-term complications.