The authors report a case of closed trauma of the thorax, complicated after a symptom-free period by acute pericarditis, combined with pleural effusion. The clinical outcome was favorable and the pericardial effusion, which was considerable at the first ultrasound scan, spontaneously recovered fully. The incidence of this delayed complication of closed trauma of the thorax is unknown. Its mechanism, related to that of the Dresler syndrome and post-pericardiotomy syndrome differs from that of initial hemopericardium which is of mechanical origin. This case highlights the capital importance of ultrasound in the diagnosis and assessment of cardiac complications of thoracic trauma.