The association of intramedullary haematoma of the left femoral diaphysis and congenital alpha2-plasmin inhibitor deficiency in a 6-year-old boy is reported. Congenital deficiency of this fibrinolytic system component is very rare and can result in severe bleeding. Clinically, an intramedullary haematoma causes pain but no swelling, and there is no history of trauma. X-rays show a hyperlucent lesion without sclerotic edges in the medulla of the diaphysis of a long bone with thinning of the cortex. MRI is diagnostic with the haematoma generating a specific hyperintense signal on T1-weighted and T2-weighted spin-echo images.